Even as a dwarf species he should have been on rats long ago, even if you were just using rat pups of similar size to mice, they have more nutrients, more mass to them that is actually body and not just fur, making them a better choice for larger snakes like that. I still would try rats, Whatever size you think is appropriate, even if you start with a fuzzy rat... get it really warmed up, around 100 degrees, and see how he takes to that.
How big is he exactly?
x2.