my new girl, Caroline, does that. often when she doesn't like the taste of pellets, or if they're too big. i solve it by offering her crushed pellets of a different kind(i have six different variety of pellets laying around), and she usually gobbles them up. :3 they may be too big for him. try crushing them up and offering them, or switch brands. flakes can be bad for bettas(they often swell up in their tummies, so make sure you soak them before giving them to him if you decide to go the flake rout), or, if you can, offer him live or frozen foods. :3 variety is good in a betta's diet!