While most BP will switch some might take longer than others, and it might require tough love.

If your BP has a good body weight try to offer a rat again next feeding day and see how it goes.

If he refuses again, same thing wait another week before offering another rat (in the mean time do not offer mice)

You can also try to put the rat in mice bedding for an hour prior to feeding time (if that is an option for you)

Also keep in mind that not all BP will switch I have a few that will have nothing to do with a rat.

The bottom line is to feed what works and is convenient for you.