How much do you feed him and how often? Have you had a fecal done to rule out parasitic involvement?
The natural functions of the gastrointestinal system will dehydrate fecal matter and urates. So, how often does he defecate? The urates being retained too long can cause this as well.
Have you tried showering him? Also, I would try feeders that promote hydration as well. Hornworms, silkworms, etc. That may help, plus, if he is constipated, feeders like this can aid in helping clear the intestines.
While trying various methods of hydration, I would still get a fecal done. IMO
Can you post a pic of him?