Having had bettas in the past that looked like that - How much is he eating? It looks like he's eating himself to death. I was foolish when I started keeping fish and desperately tried to keep bettas in my community tank - even though I killed about 6 of them just by being stubborn.
Bettas will not stop eating. It took me a very long time to realize this. If there is food, they will eat it to the point they will burst. It's happened 4 times, no matter what I did to try and stop them from getting at the food. It's like all their survival instincts have been bred out of them. Normal fish, like tetras, will eat their fill and will normally stop when they can't eat anymore. Not those pretty stupid fish. The other 2 bettas I lost because they swallowed ghost shrimp whole and choked to death. Diet is not in their vocabulary.
I have three bettas now, and I love them to death, but I keep them in their own tanks without any other fish. It's just too hard to try and feed other fish (or frogs or snails) without the betta stealing the food and overeating. I imagine that you could confine him at feeding time and let him out only when the other fish have finished, but all the betta's I've had clean the tank floor better than any bottom dweller out there - if the bettas sense food, they'll dig into the gravel to get at it!
I've had bettas with dropsy, and they get swollen and popped scales over their entire bodies, not just their stomach area. There is also the possibility that it could be a tumor, but I'm betting that it's from eating way too much. The good news is that they'll normally recover - feed very lightly (one or two peices of betta pellet) every other day until he starts slimming down. Stay away from the treats like bloodworms until he's back to normal. And if he'll eat pieces of frozen shelled pea (thawed, of course) I've heard that can speed up the process.
/www.fishjunkies.com/Diseases/dropsy.php
