I'm surprised none of the more experienced forum members have answered you yet. I'll give it my best shot.

First, unless I'm confused on my rodent sizes, I don't think a rat pup is significantly bigger than an adult mouse. Unless you are talking one that is like a weanling/small adult mouse that isn't fully mature.

I'm pretty sure that most approx 1 yr old BPs can handle the size of a rat pup with no problem. All snakes grow at different rates though. Since you say the rats look smaller than his girth, it should be ok.

You might have more trouble with changing from mice to rats than anything having to do with size.

You didn't say if the mice you've been feeding are live or f/t. If he's used to live, some snakes don't like to change to f/t.

As far as whether you should feed 1 or 2, if you think they are bigger than what you have been offering, I'd start with just 1 at least the first time. Otherwise when you try to feed again a week later, he might not be very hungry and even if the first feeding with the new food goes ok, you may get a refusal the 2nd time.