Heck I breed rats and I have two BP's that absolutely hate em LOL. They will ignore rats, hiss at them, hide from them, retreat to their hides or under the newspaper. Pop in a live mouse and WHAMMO...dinner time! LOL

As Tim said keep offering a rat first and if they want to, they'll go for it but some just never do. They are strongly imprinted hunters so there are just some that will not change prey types. I've even tried having live adult mice run around with the same size white rat pup to transfer scent. Nada....apparently BP's are no dumb bunnies when it comes to knowing a rat from a mouse (I swear Ailish actually glared at me wondering what in haydes was up and where was HER mouse LOL).

Remember he'll do just as well on mice as on rats. As long as his weekly volume of prey is appropriate for his size, whether it's one rat or multiple mice he'll do just as well on either.