Flakes are fine but a diverse diet is what most recommend. I fed my guy pellets, flakes, frozen blood worms and brine shrimp(he loved both). I also gave him a blanched pea once a week to help flush his intesties. It's suppose to help fishes with thier digestive systems. GL
Did you just get the betta? It just might take some time he'll eventually eat but a variety might help you to see what he prefers. My Apistos hate daphnia but others swear by it. Really depends on the fish more than anything else.
My bettas, in fact all of my fish, go crazy for NLS thera-a 1mm sinking pellets and same goes for frozen (thawed first) bloodworms as a treat. The garlic in the thera-a generally is good to get some picky fish to eat and serves as an immune system boost.