Do his scales make him have a pinecone look? Is it just his belly that is large, or does his whole body seem to be swollen?
If it is just his belly, he may be constipated. What do you feed him?
If he looks more like a pinecone and his whole body seems to be a little swollen, too, I would tend to think it is dropsy. Dropsy is very hard to treat and doesn't usually have a good recovery rate.
What are your water params? How do you house him? WHat do you feed him? And, how long have you had him?
Is that enough questions yet? LOL