How long has it been since he last ate? Without heat, they can't digest, so he could have an upset stomach if he had recently eaten (not sure if you could do anything about it, but it could explain some of lack of energy). If he did eat recently, I'd also let him fast (just for a few days longer than usual) or until his behavior gets back to normal. If not, then he should perk up and get back to normal once he's been warm for a while.