bettas are carnivorous.
this is why betta food flakes are so high in protien because bettas need it.
betta fish will eat flake food, but only for survival. you need to give more of a variety.
you can buy dried bloodworms, tubifex worms, and brine shrimp. your fish will eat these and LOVE them. if you want to give your betta more of a treat, you can get frozen or live food from a local pet store.
also, you can chop up earthworms and give them to your betta (gross i know, but your betta will thank you and love you!) but just make sure the pieces are small enough for you betta to eat, preferably make them smaller then the mouth, because bettas are known to take bites bigger than they can chew, and sometimes choke to death.
so if you feed any of the things above, your betta will certainly be happier, hold his tail up higher, colours will be brighter, he'll be healthier and he will be more lively. good luck!