Ball Python Feeding Problem..

[TOinfynitee]

Prince Charming is roughly 4 years old. Due to lack of proper care during the crucial first year of his life, his growth was stunted. However, when he was given to me, he was eating large mice, but that was three years ago. He quit eating them, but would eat medium mice. Now he will get the medium mouse half way eaten before regurgitating it, then he won't touch it again. He did the same with the large mice. Any ideas why?
~iesha