Well, he could be constipated. I'd try the pea thing for that. It could be depression, if it's that then every so often hold and/or place a mirror beside him for a few hours so he can "spar" with his reflection hopefully giving him some excitement and reason for eating.lol. It could be an infection or disease, there are alot out there, so first things first. Are there any odd things about him, any particular dramatic changes to his body (fins damaged, scales, so on), or the way he moves, etc... Another thing, is his water temperature within an appropriat range that stays constant, bettas become lethargic in cold water and drastic changes can easily stress them to becoming unhappy and unhealthy. Hope this covers some possibilites and helps, best of luk.