One suggestion which I haven't seen so far in this thread is:
Don't attack the tank.
Go around the tank.
Attack everyone else in the party.
Make the tank WANT to be hit.
Smart opponents are quickly going to realize that it's easier to take out the fighter's buddies than to take out the fighter. Safer for the opponents to reduce party numbers first, THEN deal with the tank.
Just like the PC's target the enemy spellcasters first. If so much of the character's resources have gone to AC, he's unlikely to be the high damage team member, so ignoring him to focus on the softer targets with higher damage potential makes sense. It is what the PC team would do, so why should NPC's be tactically stupider? Once the others are out of the way, teamwork like Aid Another against this guy becomes much more practical.
There are an array of abilities against which armor is less useful, as already cited by others.
Ultimately, I would WANT this PC to feel awesome in his defensive ability pretty regularly. If he had instead focused all his character resources on higher damage, he'd get to be the guy who cleaves through the opponents before they even get an attack and feel Awesome. He spent his resources on defense, so he should be Awesomely defended.
At 8th level, these guys should feel pretty powerful, so having town guards be a limited threat really doesn't bother me. If they're abusing that power, the town would logically take the same approach most game worlds do - hire some heroic adventurers to deal with these powerful brigands!
It's been awhile since I looked at it but...
Hobgoblin Ranger 1/Fighter 6/Barb 1:
Str 18* Dex 16* Con 14 Int 13 Wis 8 Cha 7
Feats: Combat Expertise, Dodge, Shield Focus, Weapon focus, Weapon specialization and more.
(*he started with 16/16, but has gotten a belt of Strength, maybe Dex as well).
AC 30, 33 with Expertise, 36 with fighting defensively.
CMD 28 (31 with Expertise).
Speed 50. Stealth... enough. Acrobatics is good.
Items: +2 Breastplate, +2 heavy shield, adamantium +1 Longsword, Ring of Protection +2, Amulet of natural armor +2, Boots of Striding and Springing.
To start, maybe you need your scenarios to feature more investigation and NPC interaction. That seems highly likely to be this guy's weak point, since everything he has is combat-focused. Let the PC's who invested resources in non-combat abilities shine there, and he can shine in combat.
With Combat Expertise, he's taking a stiff penalty on attack bonuses to get defense bonuses. I always thought of Combat Expertise as a replacement for Fighting Defensively, but I see nothing in the rules that prevents using both. That's a -7 penalty to hit, though (-3 from Expertise and -4 from Defensive Fighting). Why not throw in some high AC opponents and make him reconsider those bonuses? I assume he's not getting much mileage from iterative attacks.
His will save is only +1 I think, which I have used against him in almost every big combat, however the party tends to focus on enemy spellcasters first as they also know how dangerous this guy is if he gets dominated (has already killed one party member after being dominated by an Aboleth).
Spellcasters are somewhat rare, but every nation has them and every major city spots several spellcasters of higher levels. Magic items are not sold in shops, but spellcasters can be hired for jobs and may have an occasional item or two. Magic is seldom used on a large scale in battles.
So it's low magic when the player wants to complain about his weaknesses coming into play, but not when he wants to commission a very specific item for his character. Somehow, that sounds less than fully reasonable. Especially if the characters' power has also come with some fame. If enemies know this guy is almost unhittable in combat, but easily affected by mind-affecting spells, they would be pretty stupid not to hire someone who can target his weaknesses - and you've established spellcasters aren't tough to hire, right?
Hey, if he wants a bonus to Will saves, all he has to do is Rage...and suck up the AC drawbacks. I'd also suggest this organization they are up against should have an idea of the characters' capabilities (not just his PC) and the ability to equip their minions in an equally focused manner. So maybe they should start sending in, say, some Net and Trident warriors to oppose this guy. No reason they can't have feats to enhance attack bonuses with their nets. His Touch AC is pretty good, but removal of armor and shield bonuses drops him to a much more hittable range.
Net fighters staying 10' away, probably with decent AC's so he has to choose between a decent touch AC and a decent chance of hitting them (Defensive and CE being a much bigger part of his Touch AC) changes the game a bit, I suspect.