Sarah, to you. You are doing a good thing for yourself and your child. Ultimately, you have little control over what happens to your stepson, and you have full control over what happens to your own son.
The police or any medical centre can give you a referral (incl hospital) for somewhere to go and who to call for legal advice, in terms of custody etc. It doesn't sound like he would genuinely care if you took his son, but he might decide he wants to play power games and you might have to cross some hard bridges to get away from him. Hopefully he's just a deadbeat who doesn't care about custody. He sounds like a person who will only ever think of himself and you will always come off second best. You are doing a good thing.