I'm not going to comment on the dragging your son by his arm, because everything has been said and you yourself have said you were wrong and regret it.
But your DH threatened to beat you up, in front of both of your children, and frightened you so much you went to sit in the car and didn't want to go back inside.
His explanation about being beaten as a child might explain his behaviour, but it doesn't excuse it now, because he's behaved just like his father in front of his own children. Has he had counselling? Because if not he may benefit from having some.
You might have been the one dragging your son but in that moment he also became violent, to you, frightening your youngest, frightening you and with both children in tears.
Your children heard their father threaten to beat their mother. That will stay with them, certainly your eldest is old enough to remember it for a long time.
He did nothing to help you with your older son, not even step in and take your younger one off your hands.
Who knows what your DS will make of that? Will he think that you can't tell him what to do now, because his Dad will stick up for him and side with him against you? Will he think it's okay to drag people, or threaten to slap them across the room if they don't do as he wants them to do? Or will he be scared that his Dad will carry out the threat to slap you across the room if he does anything wrong in the future?
You say you are going to apologise to the children tomorrow, but your DH needs to do the same. He's possibly been more damaging to them than you have.
And you need to sort out why he just sat there ignoring the situation and left you to it. It escalated in part because you were dealing with two children at once, one being very challenging, and he just left you to it.