In my experience, it is due to the fact that he is either stressed or is not used to eating that kind of food. I first make sure that there is nothing to stress him out (i.e. cold water, other fish etc). Being that those are probably unlikely, it could be that he is just adjusting to his new home. Some bettas will eat right away, others might take a little time. If you are still worried about him, you could also buy different types of food. My bettas are used to eating tropical flakes, but they also enjoy blood worms (their favorite). Congrats on your new boy!