If your boy is 120g the rule would suggest him being fed something in the range of 12-18g. Some people don't weigh the food and go by width. You could try a weanling if it is at the lower weight or a large mouse which is the equivalent to a rat pup? Will you be able to obtain rat pups at any time? He may refuse food that is too big for him or regurge?
New BP's will roam until they settle. Once he has settles he will likely go into his hide and it is likely you will rarely see him. My pastel spent her first two nights curled behind her hot hide instead of inside it! Some BP's may just take longer to settle than others
I'm sure there is something that can be added to my post from more experienced owners grats on the new BP!
ya, she's right. but bps aren't very active creatures, the name is BALL python, cause they stay in balls