If he's already on F/T then it should not be hard to switch him to rats. Just start with a small rat that is only slightly bigger than what he is used to. One problem you may encounter is that he is use to eating much smaller food. Something alot bigger, even if its the right size for him, may put him off of feeding. Work him up slowly.

A great tip if he refuses the rat is to make sure the rat is wet and very warm. Thawing in hot (from the faucet) water and using a blowdryer on each side for a minute or so will usually do the trick. Wet rodents put off more scent for the snake. A good heat signature usually inspires a strike.