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.