If he is noticably big in the tummy, he might have dropsy. Be on the look out for "pine cone"-ish scales and a big ole stomach. Unfortunately, if he does have dropsy, there is no cure. So add some aquarium salt to his tank and keep the water VERY clean.
Other possibilities: constipation. Don't feed him for a day, and after that thaw out a frozen pea and skin it. Then break it into bite sized pieces and feed it to him. Don't feed him the next day if he still looks bad. Make sure he is going to the bathroom, and like I said, look out for dropsy.
Alexa