It sounds if he isn't warm enough and is having a hardtime digesting food. I would take the matt of the bottom and get some over head heating, they want to bury to cool off not to get warm. And if the substrate is 82 over the matt it will be abit cooler away from it, and when he comes up at night all the warmth is in the substrate and not in the air. The digestion is just as important at night as it is in the day, and white stuff on the stool is probably stuff hes having a hard time to digest. Yes I know some books say to put it on the bottom but if you were to ask some of these experienced keepers they would say its more of a last resort if you cant achieve the temps from above for some reason because its just so unnatural for them.. I don't think there is anything really wrong with him hes just having a hard time to digest properly. I would get the overhead heating and see how it goes and if still no better and would take a stool sample to your herp vet.