The reason he does that is because Bettas are little pigs.
It really depends on the size of the Betta, though, how much you feed him. Large Bettas get larger portions, small Bettas get smaller portions. I tend to experiment with different feeding amounts. If it makes them fat or they don't eat their whole portion, I feed them less the next time.
For instance, Flynn (a green Delta, who's only about 4 1/2 months old right now, and very small), tends to get 6 or so pellets, because he's a growing boy and it never rounds out his belly, and there's never any waste. Bard (a red Delta of about the same age and size), however, gets no more than 4, because his appetite isn't quite as large, and he ALWAYS leaves waste if I feed him more than that.
Hope that helps answer your question.