Couple more thoughts. I'm not sure he can actually run out of cantrips. Once he has decided which to prepare they are there until he actually decides to prepare different ones (or the ones he has are somehow wiped by other magic etc.) Hopefully he has at least one memorized that is offensive in nature for combat purposes.
If he has an arcane bond item (and currently has it in his possession or gets to it) then he also gets one of the spells in his spellbook each day to cast even if not prepared. He better make that one count if that is the case.
