Only feed him two or three pellets at a time, and no more than what he will consume in about two minutes. This might not seem like enough, but realize his stomach is only about the size of his eye.
Over-feeding betta fish is one of the biggest causes of illness and premature death. Some fish will over-eat available food to the point of making themselves sick. Chronic over-feeding will pollute the water with uneaten food and excess waste, thus allowing more algae to grow.
So take care to feed the appropriate amount of food whether you choose pellets or flakes. Feeding too much food could mean an early demise for your beloved betta.