He could be on hoppers at 75g, if you want to get his weight up a little faster. Most breeders offer hoppers directly for new hatchlings.

When I got Cake at 65g, and he was totally fine with live hoppers.

Ball pythons do stay small/grow very slowly from simply being underfed. I've seen a BP who is a year old and under 300g, from being on the supposed "maintenance feeding" aka underfeeding.