First thing I would do is dump the flake and feed him on some pellets and frozen or freeze dried blood worms. Only a small amount will do as with other fish, over feeding is a problem with bettas.
Also, since it hasn't been long since you brought him home, it will take him a while to settle in, get used to his new surroundings and the difference in water. It shocks their little bodies and some go in to a couple day depression.
Here are some good betta links.