Have some patience and stick with the rats for a little while longer. You won't harm you snake if it doesn't eat for a while. Your post sounds like you're feeding live and not F/T. If that's the case, you can try a rat with a little more movement, like a small crawler. Sometimes the extra movement will do the trick. There are also a number of other things you can try like feeding a small mouse and immediately follow that up with a rat hopper or pup. Sometimes that will get them to switch. If he just won't make the switch right now, you can go back to mice and try again when he gets larger.

Yes, some BPs remain mouse eaters for their entire lives, but that it the exception, not the rule, and many times it's more due to the perceptions of the owner than the reality of the snake.