Possibly a blockage? Have you seen him poop? Any signs of straining too poop?
I would give him a warm soak in the tub so he has lots of room to move around. Try to keep him moving if he wants to just lay still. Also feed some watery foods like cucumber, romaine, aloe, cactus to help him poop.
All this is if you haven't seen him poop or if you see him straining too.