My Axanthic was in a similar condition (age and weight wise). I started feeding him weined rats and then small rats. He gained weight quick. He went from 296g to 900+g in a matter of I would say 9 months tops. Also expect many sheds . He also ate some adult ASFs male back when I was still breeding them.

He still around 900g but he hasnt eaten in almost 3 months with the winter (or Im hoping that's what it is, my other snakes are off-feed as well) He still looks very thin, but he is so long/large now, my biggest snake. He is about 2+ years old now.

Congratulatins!!