It's very typical for young snakes that are eating pinkies/fuzzies (baby rodents) as they have tiny bones & little or no hair- they're mostly digestible. And even though BPs are often eating hoppers, it just takes a while before they need to go. Is your snake noticeably puffy above his cloaca (aka 'vent')? If not, it's just not "time" yet. They go when they need to.