I made a claustrophobic D&D character once which worked out fairly well. I also spent the time devising a guide for "How freaked out am I?" and then played along accordingly (and super freakout didn't happen unless there was no room for the character to wield a dagger). I also tended to play my character irritable and extremely short tempered when underground rather than 'totally hindering the usefulness of the party'. Maybe your player isn't as good at knowing when to play it and having it play him?