Try feeding him some peas. It may help if it is bloat. What I do d put a few peas in a small bowl with water, microwave for 45 seconds or so then transfer the peas into another bowl that has ice water in it , then after couple of minutes take the shell off the peas ,smash it a little and feed the fish. Peas are a natural laxative for fish. It helps with constipation and bloat.
How much are you feeding? You may also want to cut back or better yet after trying the peas maybe go a couple days of not feeding to see if any improvements are made in the fish.