I don't think it's very smart to not feed him until he eats rats. If he was bit, or traumatized by one, he might never eat them. They can go months without eating, but eventually he is going to start loosing weight.

I wouldn't try for more than 3 months. If he still won't eat a rat, switch back to mice. Breeding season is approaching, and he may stop eating all together. It's a long time till next spring.