It may simply be constipation rather than swim bladder. Try isolating him for 3 days without food and then feed him a cooked, shelled pea. Peas act as a laxative and should solve the problem. If you don't want to isolate him, you may try feeding the entire tank some peas. Most fish love them and it would be good for them. Make sure to remove the shells though as their systems can't digest the shell/hull.