If hes just been over-fed a little, a couple of days of fasting should do the trick. Don't feed him anything for a day or two, keep an eye on him and watch for his poo. In a healthy, active betta in a properly sized, heated tank he should be able to process and pass the bit of extra food without much problem.
If after a couple of days you see no poo and his belly hasn't gone down, pick up some frozen(NOT freezedried!)Daphnia and feed him a little. Bettas are carnivorous fish and don't digest plant matter well, so the pea treatment can often just cause more problems. Daphnia, I believe, is a water flea; a betta-safe alternative to the pea treatment. You can also feed it regularly a couple of times a week like other frozen foods.
If he seems to start getting more lethargic and/or floating on his side and having difficulty swimming, separate him out into a heated bare bottom QT, pick up some plain unscented Epsom salt, dissolve 1 tsp per gallon and add it into his QT. If he'll eat, offer him Daphnia as well. Change his water out daily with the epsom salt and keep offering him Daphnia until he improves.
Just in case he goes down that road....which, hopefully he won't.
Best of luck. :)