It's good that the swelling is going down a bit! He could be constipated. It's a fairly common condition with bettas.
Feeding a pea will help. I use frozen peas, as canned peas tend to become mushy, and the bettas don't seem interested in them. Microwave some frozen peas on High for 4 minutes. Let them cool by pouring some of the boiling water off and pouring cold water in. After they're cooled enough, peel off the skin and discard it. Cut the pea into small pieces, about the size of their betta food. They should accept it if it resembles their own food and it isn't too big. I know it's hard to cut a small pea! I just use my fingernails or a small paring knife. To give you an idea of how many pieces to feed, I give my betta 4 pellets a day, so I give him 4 pieces of pea that are about pellet-sized.
I would give the pea to the betta, and then not feed the next day. A good feeding routine is to feed their normal food Monday-Friday, feed the peas on Saturday, and then don't feed them on Sunday. One day a week of fasting is good for their digestive system.
Keep watching him -- if he gets bigger or his scales stick out like a pinecone, post back.
