Were you feeding him a different food before this? My two betta's both like the Tetramin Tropical Granules; The Rich Mix. My first one preferred flake food and bloodworms. If it's a new fish from the store, he may not be used to pellet feed, or prefer something else...
Fish food isn't terribly expensive, so it may be worth it to try something different. You can ask at a pet store too, where they can show you specific foods.