120 grams at a year old?

Ok, he's not a little underfed and small, he's WAY underfed and ridiculously small. Most baby ball pythons hit 120 grams by 2 or 3 months of age. He should be easily 600-800 grams or more. Which is to say, he's at least 6 to 8 times smaller than he should be, and you'll be lucky if he isn't stunted for life.

Being so much too small, then I'd stick to hopper or just over hopper size mice, and bump him up to a 5 day schedule. If he's eating regularly, he'll put on the size he should have already fairly quickly. If you can't get him on the size prey he needs when he needs it, then please don't keep him. He needs someone to feed him the correct prey with the correct frequency now, or he may never catch up. Once he puts on some weight, then you can slowly get his prey size and himself up where they should be.

Gale