sounds like dropsy.... if you look at him from above, does it appear his scales are pineconing (sticking out)?
Unfortunately if this is dropsy, it is almost always fatal. You can try treating it by doing several things:
-Do a substantial water change
-Raise the temp slightly (82 degrees) if using a heater. (if you are not using a heater, that could be part of the cause)
-Add an appropriate amount of aquarium salt
i agree with most of what ReMz has already said, but... not aquarium salt. dropsy is caused by water retention (either due to bacterial infection, or kidney failure) and ordinary aquarium salt will only make it harder for the fish to regulate its water levels. what you need to use are epsom salts (magnesium sulphate) i cant remember the exact dosage at the moment, but if you do a search on here for epsom salts, i'm sure it will turn up.
fingers crossed its bacterial, because if so, then treating with Maracyn or Tetracycline as ReMz suggested could cure him.
Edit: general consensus for epsom salt dosage seems to be 1 Tbsp per gal for baths or add 1Tsp per gal into the tank.