So on the off chance that you’re still reading OP, I hope you’ve not been back because you’re booking tickets home for you and your children without a backwards glance.
Firstly, physically overpowering your son and “pretending” he was going to throw him over is abusive in itself. Who the fuck gets a kick out of terrifying a little boy? Red flag 1.
He didn’t stop (and apparently you didn’t step in either
) even when your son was in considerable distress. Red flag 2.
Your son reacted as anyone who is terrified and overpowered would, by hitting out to try and get your boyfriend to stop bullying him.
And his reaction was to slap a 9 year old boy across the face.
That’s not a red flag OP that’s a row of fucking bunting and a sign writer in the sky telling you that this is a child abusing bad bastard.
I’m not an aggressive person by nature but I’d have decked him on the fucking spot and he’d have deserved it too.
What is absolutely terrifying in your OP is that you think there’s even a question to be asked. There isn’t.
Your children are stuck with whoever you bring into their lives. You are not in any way prioritising them if you are not prepared to leave him, immediately. They deserve to be protected from him.
If you stay, you’re telling them that they’re not important, that they don’t matter, and worst of all, that their own mother will stand idly by while her latest man does as he pleases.