It is fairly common for bettas to get constipated and bloated. What are you feeding him? He should be getting pellets as his staple, with occasional treats like blood worms or brine shrimp. He should only get about four to six pellets a day, broken up into at least two separate meals. It also helps if you presoak the pellets in a small container of his tank water; pellets expand in water, and it's best if they expand before they get into your betta's stomach!
To treat his current bloating just fast him for a few days. If you have any daphnia you can feed him a little of that, it helps with constipation.
I'm glad too hear his fins are doing better, his colors are so beautiful, i can't wait to see him with his full fins!