Your husband tells them "let social services claim you" so of course your son doesn't want to talk to SS, your husbands quite successfully convinced him they'll take DS away and he's scared! How the hell can you think it's even ok to ask a child to make that decision?
Right now you are teaching your children that they aren't even worth defending. You need to talk to social services and deal with this now, not keep giving him more chances and making excuses, not making it DS' choice. You are their mother and there is absolutely no excuse for you not stopping this here, it is not something precious, it is you allowing your pathetic excuse of a husband to abuse your children.
He hit your son over a fucking meal, what's going to happen in a couple of years when your son is late home? Or does one of the millions of annoying pushing the boundary things every teenager does? How hard are you going to let him hit your child?
My dad used to lose his temper when I was a kid, always at me, I was a bit of a shit. At 13 it was once every few weeks and just a slap, by 15 I was an expert at concealing bruises. I used to pretend it didn't hurt too. Then one day I answered back too many times, he broke my jaw, my nose, my wrist and 3 ribs before the dog got him off me. The fucking dog! Because a labrador had the intelligence to step in before my mum did. Your husband is losing his temper, losing control and lashing out, exactly like my father did. Your posts suggest that he doesn't mean to hit them, he gets mad and loses control, you must see how dangerous that is? how long do you think it will be before he goes too far? Hits too hard, or DS falls wrong? What if there's no one there to step in?