One thing I'm struggling is is the way he is with one of my children. And actually, in the last week, things became more challenging between he and I because I asked him to look at how he had behaved with my youngest son.
I wanted to know how things could be different if we got back together.
This is the crux of it OP.
To take him back would be to risk your son, and any other children. being damaged by this man.
To do that would be choosing him over the welfare of your children.
Whenever you feel tempted to contact him, remember that and hopefully it will be enough to stop you doing so?
Your children are viewing this relationship dynamic and being taught it's normal. And the longer you're with someone, the deeper than teaching is.
Imagine how heartbroken you'd be if your son ended up replicating your partners behaviour when he's an adult himself.
Imagine how heartbroken you'd be if any of your children ended up replicating you in this relationship dynamic.
You don't have a shared child, a shared home or shared finances. There's not one single thing you need to stay in touch about.
You can block and delete him today.
Within a year he's been emotionally abusive, verbally abusive and when he used your hand to hit yourself he added physical abuse to the mix. Because it doesn't matter whether it was hard or not, whether it hurt you or not. It matters that he felt entitled to physically punish you for disagreeing with or disobeying him.
It will escalate. He will become increasingly more physically abusive.
So, block and delete. Nothing is stopping you.
You can start to heal and use your headspace and emotional bandwidth on your children, family and friends.
You need to do that for your children or unfortunately you're not putting them first 