You poor thing.
First thing - please, please tell your parents what is happening. Imagine your DS being in a similar situation in years to come, and not telling you because 'he got himself into it'. You'd be distraught. You DIDN'T get yourself 'into' anything. Normal relationships aren't like this. You are in trouble here, through no fault of your own, and therefore your DS is in trouble too, in a sense. It's a case of family needing to pull together here and sort out the trouble - which is your bullying, evil DH. Tell them! You WILL sort it yourself - but they can help!!
If you can do that - go to them. Pack the essentials and GO.
Secondly, go to your GP, or health visitor. Get an appointment and tell them what you've said here about the way he treats you, and his threats re custody. You can frame it as you asking for information on what you should do if the worst happens and you find yourself unable to stay any longer and facing a malicious accusation. Get his threats on record.
It's all crap, you know - designed purely to have the effect it is having - to frighten you. If you left, he could accuse until he's blue in the face. SS will come and talk to you, will see that DS is happy and safe, will see that your 'worried' DH has been only too happy for you to care for the child until you left, you can direct them to your GP and HV - and the real story will be clear as crystal. Sadly, so you know what I think is far more likely to happen should you split? - that you'll find that he quickly turns his back. Getting custody? What, you mean he does all the work and knocks those parties on the head? Haha. No, he doesn't want custody - and once his lies and bullying are investigated and found to be crap, and you don't come back, I'll be astonished if he wants to take on even 50% of the responsibility. He seems to think himself too important for that!
Seriously, you do not need to fear this situation. But you do need to realise that your best and quickest way to getting this crap out of your life and neutralised is to be HELPED. By your family, and the authorities. That's what they're all there for :) to support you, and your DS. Phone your parents, and your GP. Good luck.