Without seeing him but from what you've said, I'd feed him a small rat every 2 weeks, & maybe gradually lengthen that to every 3 weeks instead. And see how he does with that- snakes don't actually do well with "reduction diets" (neither do most ppl, lol), but see how he feels & how he acts. BPs aren't very active snakes anyway, I'd imagine those large rats really weighed him down trying to digest them, poor guy! Hopefully he'll feel better eating small rats & more often. You can be flexible too, snakes certainly don't have an exact schedule in the wild either.