I'd say he's either got dropsy or is constipated.
Look above him (or at his sides as he swims). Are his scales sticking out, like a pine cone? If his scales are sticking out, it's dropsy.
Unfortunately, this can be helped with medication early on (in some cases) but once the scales start to pop out and the stomach swells, the internal organs have typically begun to fail and there isn't much to be done.
If not, he might just be constipated. Cut back on foods high in protein, and feed him one or two microwaved peas (shell them first and smash them up with a fork). Afterward, don't feed him for two or three days.
This should help.