HELP.my python wont eat!?

I have owned and bred snakes for a while but i have never had a ball python refuse food like this snake. He has not eaten in 4 months and is losing weight. He has already been checked out and is healthy in every way other then not eating and losing weight. How can i get him to eat. I have tried all ways i know.( i have offered him live, frozen thawed, pre killed and he is always fed in his cage.Please help.