Why won't my baby ball python eat?

I have a three month old male ball python that looks a little underweight, but he won't eat. I've tried feeding him a frozen small mouce, live pinkie rats, live fuzzy mice, and live pinkie mice, but all he does is smell the food, then slither away and looks around the tank or at the food. The reptile store was feedinghim pinkie mice, and he ate well for them, but it seems as though he's lost his appetite. Is there a reason why he won't eat? Is there something I can do to try and get him to eat? Will he starve if he doesn't eat soon because he is an underweight baby?