It is normal for a betta to go through a period where they won't eat for a while after being put in a new home.
Also, if your kit came with flakes, the flakes may not even be specific to bettas and he may not like them. If you think it may be the food, then pick up some new food, preferably betta food pellets (try Aqueon, Omega One Betta Buffet, or New Life Spectrum). It is possible that your betta will like betta specific flakes, but I think you will see the best results with pellets. Just to reiterate, if he doesn't take the new food immediately, just give him about a week and eventually he will come around and eat. Just make sure not to leave food laying around for too long, as it will pollute the water.