Well, I'd ask what the problem is if he's not showing any discomfort.
Bettas, when full, should show a little bulge.
this guy is pretty full and i'd stop feeding him... but nothing to worry about:
http://s4.hubimg.com/u/222575_f520.jpg
Now, this guy... that's bloated:
http://i12.photobuck...h/sickbetta.jpg
What are you feeding him? If it's pellets, they tend to expand after a while... perhaps try soaking them before feeding him.
Edit: this is a good example: http://www.bettatalk.com/food.htm