Some bettas are very picky eaters, even if it kills them. Since it's been days and he still won't eat, I suggest you get some frozen bloodworms. Don't get him used to bloodworms because he will never eat the pellets. The bloodworm is only for back up . . . . say 2 days of pellets and one feeding of blood worms. That way he will always be hungry but will not die. Eventually he should eat the pellets. Once he does, NEVER give him bloodworms for at least a month. Then give him bloodworms only as occasional treats.
Remamber to remove any uneaten pellet after 5-10 minutes.
If he won't take bloodworms, your water might be a bit too cold. Try raising temp. If you don't have a heater, place his tank somewhere warm.