This is either an indication of the beginning phases of dropsy or internal parasites. There are meds you can try if it's parasite, but I'll need to look up and find the right combination for you. Bettas are sensitive to a lot of meds and a lot of the common meds don't work with the bettas. If it turns out to be dropsy, it isn't curable, all you can do is keep the fish comfy until it's time comes. The meds won't hurt if it is dropsy, so it's worth the try. I'll post back later after I've done some research on what best to use. I've dealt with this in many fish, very few lived beyond 6 months from this stage of illness, and only 1 ever was relieved of the bloating. Keeping the water extremely clean is the best
thing you could do, and keep it warm between 78 - 82. A 1 - 2 gallon water change every 1 - 2 days would help a lot and may help increase the appetite. Have you tried to feed live adult brine shrimp? This would be very healthy and very tempting to get him to eat. If you can get him to accept the live brine, medicating him would be much easier for you, too.
I'll be back with meds instructions later today/tonight sometime.