Yes I understand, it's very very hard to move on knowing that at any point, your abuser could come back onto the scene and bring with him some kind of power to see your son.
That is an awful way to feel and I sympathise. In some ways it becomes so much easier as your child grows and you don't see his father from day to day. The more time passes the better and safer you feel. I know that.
Your son is still very little. The fact that you stood up to and left this abusive man means that there is less of a chance he will try and make contact with you - he knows you won't be beaten that easily. You've proved you have self respect and bullies don't choose victims who have self respect. not when they could choose someone without any.
I think a solicitor would be a good step - to find out your rights, your options, what to do if he does turn up one day. just so you have a bit of a plan, and someone on your side.
You ought to get legal aid for a consultation on this, it's what I did and it made me feel better.
I didn't tell mine where we moved to either, but then the phone company mistakenly published our details in the phone book when we had always been ex directory - we got compensation - and he found us that way. But he hasn't been round. Just a card that year and I got a mutual friend to tell him it had freaked me out, and please not to do it again. He hasn't.
I don't think you owe your son contact with this man until your son is much, much older and able fully to understand what the story is. I am thinking early teens at the soonest. You certainly owe your ex nothing, not to see or be with his child.
Speak to Womens Aid for advice. They are great, can even get you free legal advice too. And go from there regarding protecting your own and your child's future from this man. He sounds a coward, most likely lazily blaming you for being an evil witch, to all and sundry - well as long as he stays away it doesn't matter.