He is a wee bit skinny. The advice in this thread is spot-on. His back story is super sketchy though, thank you for stepping up to the plate and seeing he has a good home!

Don't feel pressured to fatten him up quickly--these guys are slow healers and slow growers.

Babies should typically be fed every 10 days, whereas juveniles are fed every 2 weeks. He appears to be a baby in my opinion based on your pics... Try F/T rats the size of the snake at his widest. When the snake eats, there should be a barely discernible lump in his body. If the lump is big or bulging, the prey is too big.

Thanks again and good luck! Feel free to come here as often as you like with questions and updates!