He might be just being stubborn with you and you are "in training." He might like the Hikari pellets because that is what he knows. Have you ever given him any treats such as bloodworms or
brine shrimp, and if so, did he eat them eagerly? If so, you're definitely being trained by a fish.
Some bettas will appear to start to eat, chew, then spit out the food they don't favor. It's not really regurgitation. If they're healthy, they're just trying to train you to keep up the favorite treats. Keep at it with known nutritious food, and if he's hungry enough, and if he's not ill, he will eat the pellets.
I hope he's not sick, and just being a picky betta.