Let him fast for a couple of days. You can feed him some frozen Daphnia (water fleas). These are full of good roughage that helps constipation in fish. They are more likely to eat these than peas, since bettas are carnivorous.
Get some peas. Thaw them or microweave them to where they are soft. After that squeeze the outer skin off so its only the mushy inside. Plop it in the tank for a hour or less and see if he eats. Is the fish swimming on its side or floating to the top constanly? becuase this is it then its most likely Swim Bladdar Disease which you do the same thing but just so u know the name.
Just offer it some Veggies. I give my fish (and Betta's) cucumber once a week to help them deal with constipation. You can also boil Zucinni too. 1 Cucumber feeds all my aquariums (15).
Feed him some shelled pea, and lay off on the feeding for a bit. You can feed them every other day, or let them fast one day a week.
Give it live or frozen daphnia to eat it will ease his digestion.
green tea
