My best advice is patience. If he constricts but then ignores the rat, give him his mouse, and then try another mouse-scented rat again next week. It can be quite a hassle, and seem like a waste, but some snakes are very reluctant to change--the fact he was willing to strike and constrict the rat is a good sign. Try to offer a baby rat the same size as a mouse--this will seem less intimidating and more familiar to him. It may take a few tries, but eventually, he will eat it. I personally feel it is important that he get his meal regardless--it's positive reinforcement associated with the presence of the rat, even if he doesn't eat the rat itself.
Once he accepts and eats a mouse-scented rat a couple of times, reduce the scent--just give it a quick rub in mouse bedding. If he has no problems with that, only then offer one that is unscented. Once he takes an unscented rat, you will most likely have no further issues, and can start increasing the size of the rats.

Note that some ball pythons are just that finicky, and will try a new food, and then stubbornly hold out for the old! I have a few balls that insist on switching between rats and mice--they will take rats sometimes...but not every time. Others will take F/T only occasionally, and insist on live half the time. I have had best luck in getting a ball to eat what I want when it's been eating at least something on a regular, consistant schedule.

For some, it's just a matter of learning that the food you are offering is actually desireable prey...but personalities and preferences vary, and there will always be a minority of snakes that will try the new food item, and decide that they simply don't LIKE it.