I was going to suggest a vet. Maybe parasites, or some other trouble inside (do snakes get cancer?).
Are SURE the temps are good? Just wondering if maybe you have another thermometer to double check, incase the equipment is malfunctioning.
My third suggestion is a long shot, but everyone seems to be at a bit of a loss here anyway....Do you have a separate enclosure or tub (or another place in your living space) that you could move him to? Maybe something about his current enclosure, the people can't sense, is bugging him.
Good luck.