Sounds like he has a major strength with a significant drawback. Seems balanced to me.
And it has been, for some time. The problem is challenging him in the area he is good in, not finding something he sucks in and just exploiting that weakness as much as possible. That's not fun for the player.
So how can the PC's pick and choose a customized set of magic items exactly matching their desires - in this PC's case, focused on AC maximization? Seems like they have an advantage the rest of the world largely lacks, so why would you expect threats lacking that advantage not to fall behind?
Item Creation feats of course. The party can make everything they have, except items containing Mithral or Adamantium (these have been found fairly late).
I give whatever items I like to the NPCs too - which has a tendency to end up in the player's inventory. They don't generally have so much of course, because I base it off NPC wealth, but there have been exceptions.
So why don't the enemy have similar access to spellcasters to craft magic items for them? Starting to sound very little like a "low magic world", just one where most spellcasters make items rather than risk their own skins.
They do. And the campaign is low magic only compared to normal D&D where it is assumed any PC can get any item worth 100k gp or less in a major city. In my game they have to make it themselves or find someone to do it for them.
Again, not seeing the unbalance. He's very effective against mooks and otherwise not so effective, so how is it fair to take away his advantage in the one area where he really shines?
He is invulnerable to mooks, he is very effective against tough enemies, and he is the only one able to stand up to the really tough melee monsters (the rest would be crushed fast). He's (almost) doing his role as a tank (can't draw enough aggro because of high AC and meh damage).
Conan doesn't get dominated once (for several days, killed a party member), Held 2-3 times, blinded 3 times (glitterdust), Feared 3 times. He's not focused on AC and defense against mundane opponents, like this character. He also doesn't have half a dozen or so magic items custm built for his specific needs. He lives in a truly low magic game (where spellcasters are not PC's).
Conan lives in an extremely low-magic setting. He gets dominated occasionally, but always manages to beat it off in the end because he is the hero. He's blinded a few times, both mundane and otherwise. He is Feared.. nah Conan knows no fear. Conan often fights unarmored, and manages to parry almost everyone, so he is definitely good at AC (probably uses the Defense bonus house rule from unearthed arcana

)
When is the chart used determined? If the character is a W3 and decides to take L4 as a rogue level, which chart does he follow and how much xp does he need to advance from L3 to L4? What about half casters? Medium chart? Do paladins and rangers count as casters?
The idea was that you choose one at the beginning of the game that limits you to a certain class. Paladins and Rangers (and Bards) counts as half casters, using medium chart).
Option One: Throw Tanglefoot bag at high AC PC
Option Two: Use magic that forces Will Saves
Option Three: Use mooks! Even one flank buddy who used Aid Another add +4 to hit!
Option Four: Use Sunder
Option 0: Check to see if you have anything new to contribute.
+2 Breastplate - 4000 gp
+2 Heavy Shield - 4000 gp
+1 Adamant Longsword - ~5000gp
+2 Ring of Protection - 8000gp
+2 Amulet of Natural Armor - 8000gp
Boots of Striding and Springing - 5500gp
Total cost of listed Items: 34500gp
Level 8 Suggested wealth for level 8 : 33000gp
Assuming he has no other equipment, has never spent any money on consumables(potions, scrolls, etc), and has never had to bribe or hire anyone then your still a bit over
Sure its just a suggestion, but it's also there for player balance when taking magic items into account.
Hmm haven't been keep that careful track. It should be noted we just slowed down the campaign to slow xp progression, so he should be level 9 already (meaning he would be below reccomended wealth), but now I need to hold it off for awhile for that character.
Still he's been wanting a Mithral Full-plate since level 5. They don't get anything they want.