Hi welcome to the forum, this is the best site to get the right info on your bp.
Follow the Ball Python Manual and you should be good.
at 15-16" he can eat a full size mouse with no problem and that is what you should feed him.
Try not to handle or feed for about two weeks to let him get use to his new house and when you handle he shouldn't defecate on ya. ( was more than likely urine than feces) he was just stressed more than likely.
Any other questions or concerns please just pop on and ask the people on here are awesome!