I have no idea, it could be some kind of growth like a fungus and he's spitting up his food because he feels bad....I'd go over to
FishForum and post the question in their Freshwater Emergencies and Diseases section. Be sure to include in your post the size of your tank, what the temperature is, what filtration you use, and what your water parameters are. Be as detailed as you can regarding what the 'bubble' on his belly looks like and post a picture if possible. Someone over there can probably help you.