Not sure why you are concern an male BP even as an adult does not require anything bigger than a small rat (55/75 grams) which you are already feeding anyway.

If you want to switch you need to switch to same prey type not go from live rats to frozen mice.

It's one step at the time and f your BP already take rats you might want to stick to rats to avoid credit multiple preys in the future.

Swigging can take time and tough love, be patient and if the animal refuse to eat do not offer anything else.