Maybe his growth as slowed down now. How often are you trying to feed him? If weekly, wait 2 weeks before offering. If he refuses after 2 weeks, wait 3 before you offer again. Offering too soon just stresses them, & won't help you, especially if his body is telling him it's not time to eat yet.
What is his body weight? It might just be too much food for his size. Be patient- snakes don't usually get such regular meals in the wild as we give them in captivity.