Do you soak the food in some tank water first to soften the pellets before feeding him? Sometimes they’re too hard for them to eat and they’ll discard it. That or mine will spit it out multiple times to break it in to smaller pieces to eat. I would try hikari micropellets because they’re quite small.
I noticed my first betta did that. He would take the pellet, spit it out, then take it again. I think they do that to help soften it and break it down. Try giving him a variety of food. I feed my new little betta a variety of flakes, bloodworms, and daphnia. Sometimes they can just be picky eaters