One little thing I think most over looked in this situation. You changed his diet!!! . though ball pythons are certainly not the hardest babies to get to feed after hatching they generally have a preference for either mice or rats it very well could be since the breeder started this little guy out on rats ..... that he wants a rat. btw a pinky mouse is definatly not large enough to sustain a growing hatchling
try him out on a nice fat warm rat pup and see how he likes it