When a Betta eats veggies, it should always be a "snack" and not take the place of their regular food unless they are ill or constipated. Only on very rare occasions can they have a Pea Fast when they are totally constipated...
This means ..
No food for one whole day and then peas only for 2 or 3 days before going back to their regular food...2 small (very small) meals a day. (2-3 pellets OR 5-6 worms no more)
This has to be a ONLY when necessary thing though as bettas are carnivores (meat eaters) and have to have protein every day unless they are ill. If you feed them only vegetables they will not get the nutrients they need to live. BUT they do have to have the peas or some bit of veggie two times a week at the very least. This is like a third meal when they get it not to replace the food they would ordinarily get.
He can have peas every day if you want to prepare them for him, but they need to be in tiny pieces and only 2 or 3 pieces or you will make him sick. Just cannot be in place of other food. He has to eat every day something that is for his fish diet too. Just very small amounts, his tummy is only as big as his eye and he cannot overstuff or he will eat himself to death.