This is not metagaming

The only AC increase that is not immediately visible in this case is from the fighting style, and that is a +1. All others are readily apparent in the game world, unless measures are taken to hide them (eg. disguise self). Heavy armor and shields are quite obvious and even low-intelligence beasts recognize that they have a hard time biting metal. Adjusting tactics based on this information is completely fair.

Even in other, more nebulous cases, like mage armor or Unarmored Defense, the fact that their attacks are not hitting the target is obvious. Just like players, NPCs and monsters can know when they roll 17 for an attack and miss a seemingly unarmored opponent. The communication of this information and adjustment of group tactics is dependent on the intelligence and training of the combatants, but they can draw the conclusion "this guy has high AC", even if you would phrase it differently in-game.

Evaluate the information at hand and discern whether or not you are metagaming.

You should make yourself aware that those 19 AC come with a lot of resource investment not only due to the choice of the fighting style that defines the character, but also the choice of character class. During the early encounters, you will often face enemies who will have a tough time dealing with a high AC (yes, + 1 AC and the option of proficiency due to the fighter class are an investment). This is the player's time to shine and enjoy that investment of their character-build.

How will that affect the monsters who deal with the tanky fighter? This heavily depends on what kind of creatures you will throw at them. A Zombie (MM 315) has limited ability to reason and will attack the closest enemy, but other creatures aren't that limited in their reason. A Harpy (MM 181) would never go for a fair fight or face an obviously heavy armoured warrior head-on. Creatures have varying intelligence and tactics that you can usually find in the descriptions of creatures. Some of them will tackle the heavily armoured fighter head-on (letting the character building decisions shine), and others will immediately notice that the fighter is not the target that they will want to attack.

That creatures make choices gives your players the option to play with tactics and all the interesting options that encounter-solving has in wait. If you force an intelligent creature in a narrow pathway for instance and it only can reach the heavily armoured fighter or make the decision to slip past risking opportunity attacks (PHB 195) to reach the caster who erupts lightning from their palms, then that choice and that player set-up pay off.

Threat assessment as long as it uses the information that is obvious and discernable to the creature is not metagaming. The player group reveals by their actions and their equipment what kind of threat they pose, and intelligent creatures will reassess the threat any player character poses.

So how do you avoid metagaming further information that you as the GM have, but creatures do not have?
This is done by clear communication and description of what your player characters want to do before you narrate their actions (PHB 5), and reflection of the information that is accessible to the creature. Sometimes you accidentally metagame particular things that wouldn't be obvious to a creature just like most players know things that their characters don't (yet) know. I consider that to be a wash unless it becomes problematic, but that would be a different question to ask.

Plan how monsters will react to this sort of information!

1. A berserk monster might attack whoever's nearest indiscriminately.




2. An animal hunting for food might pick off someone from the edge of
   the group.




3. Goblins might rush in but avoid whoever looks scariest.



4. 
   

   Mercenaries, experienced fighters, might have thought-through
   tactics, e.g. "take out the glass canons first" or "all attack the
   same enemy".

   
   
   

   For "average" enemies I often have them attack the closest player, as a sort of default, so if the players play smart and the vulnerable ones get back, the brunt is usually borne by the tanks, but if the players get into a muddle where they're not supporting each other, the squishy characters can get in a lot of trouble.

   
   

Mix it up. It affects an encounter with a 'monster' as much as its stat block does. If you describe it appropriately, no-one will find it out of place.

This is one of the things his character specialised in, so the character should be good at it. Ideally that will come up fairly often. But the enemies don't always have to attack the tank, if the players don't make an effort to have that happen.

Decide what an appropriate mix is, choose an appropriate variety of enemies, and the individual combats should take care of themselves, with some where he shines, and some where it doesn't help.

Having Your Cake...

The other answers make a good point for this, but it bears repeating: The whole point of the game should be to keep your players engaged while telling a story together. I find the best way to do that is to focus on the PCs' strengths and make sure everyone has a chance to shine. They don't have to shine all of the time, but no one wants to feel like their character is useless and just along for the ride.

In this case, your player has settled on the high-AC tank for the kind of character he wants to play. This is honestly more of a concession on his part than it might look at first. Sure he is harder to hit, but he also gave up a lot of the options that would have upped his damage potential. I would argue that a character optimized for damage is much harder to balance than one optimized for defense so this actually works out better for you.

Letting the tank shine is a simple enough task. Throw in some physical-damage-based monsters and let him stand in front of them like the walking wall he wants to be. Play up how dangerous the monsters are, describe them chewing on his armor but not penetrating it, and your tank is going to feel like an important member of the team that carries his weight.

And Eating It Too

It sounds like what you are really getting frustrated by is that you want challenging combats (entirely reasonable) but the tank makes that difficult. Either you focus on him and feel like you are cheating or you ignore him and feel the same way. The good news is that it shouldn't be too hard to create scenarios where the tank is meaningful but not disruptive.

My very first suggestion would be to get used to creating combat encounters that look like boss fights in games. You want one big scary obvious threat, and then some number of smaller minions for back up. This way your tank can handle the big guy while the rest of the party mops up the minions and then focuses on the real threat afterwards.

A really good example of this for Level 1 characters would be a group of goblins led by a Hobgoblin (or bugbear, or chieftain, or whatever else). The hobgoblin is going to stand out as the main threat when surrounded by the smaller goblins. It should be fairly obvious to your PCs that the tank should handle that, because the mage would be turned into paste if he tried to do so. While the tank and the hob have a very epic (and slow) fight the rest of the group can pick off the goblins. Then everyone helps clean up the hob as they get the opportunity.

Switching Things Up

So when you want your PCs to have a standard brawl it is really easy to come up with an encounter where the tank shines but doesn't ruin everything. But that kind of fight would get boring fast if it was the only kind, so lets look at what else you can throw at the party and how the tank plays into it.

• 
  Magic attackers - If you actually want to do damage to the tank, or threaten him in general, this is the way to go. Magic Missile doesn't care about his AC. Fireball hits his Dex save instead, and also negates his just standing in front of the rest of the party as a human wall. Think about how big of a threat your tank is at the moment when deciding if the enemy would use their spell or ability on him. Smarter enemies might choose to go after weaker party members if they have the choice, while low-Int enemies might just see a big shiny meatshield and get tunnel vision on taking him down.


• 
  Ranged Attackers - This is similar to above in that it negates the tanks ability to block damage to his team, but is less likely to be a threat to the tank himself. Ranged attackers have much more options when it comes to choosing a target in combat. It doesn't really matter if the tank is the closest enemy to them as long as he is not the only enemy in their range. This is actually a really good option for you to make challenging fights, because you can change how dangerous the fight is on the fly just by choosing to have more or less enemies target the tank.


• 
  Dangerous abilities - Also similar to the magic attackers point, there are some enemies which have extra abilities which make them inherently scarier to fight up close. Imagine a pack of Ghouls attacking the party. Even if the tank is hard for them to actually land a hit on, the effects of just one hit is much more pronounced. Enemies which have any kind of affect on hits other than just damage will add an extra edge to combat. They will also be a really obvious draw for your tank, because letting a squishy mage get hit by them would be even worse. Just be careful not to make these enemies too dangerous, since one slipping past the tank could really mess up the rest of the party if they aren't prepared.

Legos, But With Monsters

Using the handful of enemy types above you should be able to build a wide variety of combat encounters that keep things interesting for the whole party. The tank can handle the big burly fighters, or hold off the ravenous ghouls, while everyone else can deal with the other threats. If you want to injure the tank just shoot him with magic. If you want to ignore the tank then get a group of bandits with crossbows. The more of a threat the tank looks like during a fight, the more enemies should target him. You can build your fights however you want or need to, with just a little forethought.

Make sure everyone else has a chance to shine while the tank holds off the stuff trying to murder them and I guarantee your player will love every second of it.

Others already have some very excellent answers (would make this a comment if I could) to this question but no one addressed this feature of 5th edition; bounded accuracy.

While your tank is hard to hit at low levels, after a few levels it will become less of an issue as monster attack rolls tend to scale up faster then player AC does. The PC's don't stay at first level for very long.

• Example: An Orc (CR 1/2) has 16 Strength, so has +5 to hit. When it hits, it
  doesn 1d12 + 3. On a crit, 2d12 +3. All an orc needs to roll is a 14
  or better and there's a chance to one-shot this tank. The PC can
  try to evade that orc, but the orc's aggressive feature allows them
  to close with nearly any enemy. While well armored, your example PC is hardly invulnerable.

Anecdotally I have found it harder as a DM to give high defense characters any time to shine as monsters can literally just ignore them and attack the squishier targets since the tank has no way to actually force enemies to engage them.

Also if they get surrounded, monsters attacking with advantage, or just rolling a lot of to hit rolls, will frequently get past the tank's AC.

Even though the other answers are correct, I don't fully agree with them.

Yes, You should avoid falling in the "DM vs. player-trap", but I think you should play the monsters as they would be playing themselves. They will take every advantage they can in order to stay alive or to win a fight. That's their instinct. If you don't do that, the players will notice, and this is also metagaming.

To answer your question:

AC translates to physical properties which are not meta

An AC of 19 means that the creature will look very heavily armored and very tough.
The monsters will often attack the closest target they can find, but they might also attack a different enemy that is doing more damage to them, but looks less armored.

Same applies to PC's when they see creatures: some will look small and weak, others will look very tough. They can expect that the creatures that look weak have a low AC and HP, and the tough creatures have a high AC and HP, even though they don't know the number because this would be meta.