My betta looked the same after GF's mom took care of it for the summer. The problem is severe overfeeding/constipation. The betta pellets contain ash and are harder to digest, and they also expand in water. What you need to do is starve him for ~3 days, then feed him a cooked pea, which is a laxative. The tummy should shrink dramatically. Maybe also vary his diet a bit with live/once live foods.