Yep, it's like Roy, but not smart.
Rust monsters have been mentioned, as have touch attacks, but consider disarming the shield. That's always fun. Surprise attacks (he sleeps in armour? start leveling fatigue penalties) at night will catch the player unarmoured, tripping the player will reduce AC by 4 (prone, plus, tripping is a touch attack), grappling monsters don't care about anything other than touch AC, pit and floor traps don't care about AC, neither do many magical traps...
Seriously, though, this character has invested a lot in his AC, hopefully, so let him enjoy the fruits of it. If the party is creaming encounters because of his AC, just up the amount of enemies so while he's tanking the pirate captain everyone else still has to deal with the ship's mage, the corsair crewmen and whatever else. i.e. you can always avoid his AC, by using alternate attack forms or mobile enemies who move past him to strike other party members, but don't. i.e. walk the line between letting him dominate encounters by taking all the hits from the monster(s) and making him feel useless by not letting him do anything, by designing encounters where he can be useful without dominating the show. Easiest way to do this is more enemies so he can block/tank some, but not all, or enemies he has to chase around the area (while benny hill music plays with him chasing the monster chasing the wizard) etc etc.
