Don't feed him a pea, it'll just give him gas, the last thing he needs. Fast him, a betta can go weeks without food, but you shouldn't need to that long, and look for other symptoms that might indicate a tumor or parasite. From the picture, it looks like the worst bloat I've ever seen, but it could be something else.
After looking around some, it might be an early symptom of Dropsy. The BFF entry for Dropsy says this:
•Symptoms: Your betta will have a bloated belly and raised scales. They will look like a pine cone. This is usually a fatal disease caused by an internal bacterial infection resulting in internal organ failure but many have had success bringing fish back when treated quickly.
Early Symptoms: Swollen eyes (important), Gray belly (important), Clamped fins, Lethargy. If your fish has swollen eyes and a gray belly, I suggest that you treat it for Dropsy.
•Treatment: If you spot the early signs of Dropsy then treat him/her with ES at 1-2tsp/gal and Jungle’s Anti-Parasite pellets while performing 100% daily water changes. It helps to increase the temperature to 84*F. If he/she has begun Pineconning then do the full course as described below:
Performing daily 100% water changes. Increase the temperature to 84*F. Add 1-2 tsp/gal Epsom Salt. Use API General Cure OR API Erythromycin OR Maracyn II and/or Maracyn for best results. Feed something containing Metronidazole, for example, Jungle’s Anti-Parasite pellets. If caught early, Dropsy is curable.
There are other sites that have more information, and I found this one the most informative.
This was helpful, as well
You should do some searching on your own though, and look into finding some antibiotics.