I'm actually surprised he's growing fast on that diet... You're doing the right time span but not really the right prey. New hatchling BPs can take small HOPPER mice. Move yours up to a fuzzy, and if he takes that keep him there for 2-3 feedings and move up to the small hopper. Prey should be about the same size around as the snake at the widest point.

Every snake has it's own growth rate. Some grow fast as babies and seem to stop, others grow very slow to a point and then go into growth spurts.