Try starting him up rat pups first. Then move to weanling and so on.
Basically, start him on something helpless to build confidence and move up from there.
It may also be that he's not intimidated by them. Alot of times they shy away from food they don't recognize. Pre-scent the rats with whatever you were feeding your BP on before. It worked for all but 1 of mine...
I've been able to successfully convert all of mine over to rats using the "tough love" method. If they have good weight on them, they only get offered an appropriately sized rat once a week, and are never offered a mouse.
A happy, hungry ball python will eat what's offered to them. They just have to get hungry enough. I've not had any take more than four weeks to finally take a rat. They may refuse again the following week, but they will usually eat again the week after that and from then on out.