His touch AC is 15, which ironically may still be the highest touch AC in the party (the others being 13 and 14 I believe). I don't want to ask him to play another character, this is a recent issue to a character he's played for over a year now.

I could throw some AoE stuff at him, though he does often go for spellcasters first to take them out quickly with his hammer or a fireball, as does the Cleric, so sticking them somewhere difficult for even spellcasters to hit them may work.