When i first read your post, it sounded like your fish has a tumor. But now, it could be a number of things.
Is your fish like that all the time? or does he have days where the swelling goes down?
If it subsides some days, it may mean that he is becoming bloated from the food you are giving him. If that is the case, then you might want to change the food you are feeding him.
Also if you feed him every day with lots of food, I hate to say it, but thats ONEEEEEEEE FAT FISH!
Your fish could also have a swim bladder problem, but that would involve swimming difficulties, such as floating to the top (being unable to swim down) or being unable to swim to the surface for oxygen.
Hope some of this helps, and let me know some more about him
~*Lauren*~