The problem could be that rats and mice and a completely different smell so he could be avoiding the rat because it is not a mouse. You could take one f/t mouse and one small f/t rat, get him to take the mouse and while it is going down right as the tail is going into his mouth gently place the rats head in the snakes mouth. Only do this with a really small rat. I would take a medium mouse and a rat that is slightly lager than the mouse. Get him feeding like that a few times to get him to weight and then switch him to just rats.