Beta fish are not big eaters. You should feed him once or twice a day. So if you feed him more than that he might be full when you feed him.
Or He might be nervous since he is new, so if he is in a bowl with more fish, put him in a separate tank. He could just be getting use to his surroundings.
But if he is in a tank alone, and he is not sick, then try some different food. Betas can eat regular fish flakes, blood worms, or beta food, which can be found at any pet store. (and if you buy them blood worms than be sure to keep them frozen. i know because i had an aqua frog that ate those.)
Anyways, make sure he is in a tank alone, you feed him right, and he is not sick. If this doesnt work, change his food.
Good luck with ur beta!