I've heard it suggested that you get a mouse and a rat of about the same size, which together make an appropriate sized meal for your bp. You let the bp eat the mouse first, and just as he's almost down swallowing ( right when there's just the tail left or so), you put the p/k or f/t rat in his mouth and his feeding instinct should be strong enough to result in him swallowing the rat as well. Once you've done this a few times, just try feeding him rats alone and he should start taking them, having become accustomed enough to their taste and smell.

Good luck!