This boy is a rescue and my first ball python, I’ve previously only had corns and garters. His past owners kept him in filthy conditions without enough heat and told me they were only feeding him 2 hopper mice every 2 weeks. The problem is, he will be 4 in July and I checked his weight and he is only 295 grams!!! He isn’t even 2 feet long. His owners also said he was bitey and only ate live. I convinced them to give him to me… well not only is he a sweetheart but he took f/t adult mice like there was no tomorrow. I feel hes lucky to even be alive! They were my brothers ex roommates, my brother is the one that told me they had a neglected snake. I’ve had him 2 1/2 weeks and gave gotten two meals into him feeding a week apart. Someone suggested I try to switch him to rats to help get his weight up. My questions are, with him being so stunted how much can I expect him to grow at this point? He should already be almost full size. Will he be small forever or does he have a chance to catch up? If I should switch him to rats, what size should I attempt first? Aduld mice seemed like a good size but after 2 feedings he looks like he could take a jumbo mouse, I dont know the rat equivalent sizes. And how many grams over what length of time is a safe speed for gaining weight? He is a lot more active now and likes to explore. Definitely has a lot more life in him than when I first brought him home.
Good for you for caring enough to rescue him. Yes that’s pretty small for a 4yo but he looks like he should be able to make a good recovery. BPs can easily live 15-20+ yrs and will continue growing their entire lives. He should still have plenty of time to get to a nice adult size. Males generally are/should be smaller anyway so that shouldn’t be an issue as long as there aren’t any underlying problems.
As for feeding, the general rule of thumb is feed something approx as big as the snakes thickest part of it’s body. If he’s taking lg mice then a couple rat pups or a weaned rat should be a nice meal. He’ll probably jump up to smalls pretty quickly. If you’re trying to switch I would suggest trying pups the 1st time. Don’t speed feed, just stick to proper size 1x week and he’ll be fine and bounce back quickly. Often times trying to help heal a reptile to quickly ie; speed feeding etc can be more stressful and cause more severe problems.
Goodluck and continue asking for any help you need here. Post some pics of his progress.
Thank you! Yes I will try to use rat pups first. I will definitely be sharing updates 
Oh wanted to mention, I fed him one mouse at each feeding, spread a week apart. Not 2 mice per feeding. Sorry if I worded that weird.