The best advice I can give you is to switch out the pure radiator heat for a heat pad glued to the side of the tank. The frog may not appreciate the entire tank being one temperature. They like a nice temperature gradient

one more question: how big is the frog? (Length from mouth to vent and width)

if the heating changes don't solve the problem i would definitely recommend getting a fecal exam done by a herp vet.

If he is maintaining a good weight and no parasites come up in the fecal exam, I really wouldn't worry that much. He may not need a lot of food to maintain his weight.