hmmm...I wonder if he got the taste of mice in his mouth and now won't touch rats? I have a male spider BP that will only eat mice, but every now and then I can trick him to hit a rat...but after he does he acts like its' the worst thing he's ever eaten. Maybe:

1) Let him go a week without eating
2) Rub a rat with lots of mouse-flavoring (aka rub a nice mouse all over your rat...and by nice...well, I guess any mouse will do) and see if he'll eat the rat.

I believe a male BP can go his whole life just on adult mice, so if he doesn't switch back it should be no problem.

JonV