Go to WalMart or Target and get a kitchen scale that registers weight in grams as well as ounces. Figure out how many grams your BP weighs, then weigh a feeder mouse. You can feed a young male BP 10-15% of its body weight every 5-7 days until it reaches 500 grams, then one small rat every 7-10 days.

When he gets older he may decide not to eat for a few weeks or months, which normally causes more anguish to you than the critter.

Also, if your pet store sold you a female - shops mis-sex snakes all the time - she may get big enough to take a medium rat after a few years.