How big is the snake and how big of a tank did you put him in? What are your temps and humidity? Also, when did you change him to the new tank and how long did you wait to offer?
It can be difficult to get picky mousers to switch over to rats, so first things first, make sure your new tank is set up properly and is an appropriate size for your bp. Once you move the snake, it would be a good idea to wait and let him acclimate to the new environment before offering. It could be stress related refusal, it could have something to do with the setup, or it could just be that the snake is used to mice and the rat has an unfamiliar smell.
How big is the rat in comparison to the mice you've been feeding?