@RainingBatsAndFrogs
This is very hard OP.
Is your H like this with all the kids or does he particularly get on the case of Ds?
Growing males often clash with older males in the family. It sounds as if your Ds has been pushing boundaries, and was escalating to being rude to you. He needs to learn decent boundaries.
And your H needs to observe decent boundaries.
But I know two nice happy successful families where at a certain point a Dad and Teen son came to blows. It didn't happen again.
I wouldn't kick him out atm but I would demand that he talk seriously to DS, apologise, and your DS, whilst knowing that his Dad should not have grabbed him, should know that he should not have been rude to you.
This. You need to give both 'men' a way out, accepting they are who they are. It's all very well for you to say you just ignore obnoxious S, but that has trained him not to care if he is rude to you, and not to care if the household is disrupted by him and his dad having shouting matches.
What is alpha male supposed to do when his mate is being attacked? He has confined himself to the shouting matches previously, but he knows what goes on in juvenile male brains. You have taken the luxury ignoring, walking away, in other words of letting "obnoxious " S learn he can get away with insufferable bad behaviour. But family dynamic says dad can't let it escalate, and he is right. Grabbing the boy's collar and threatening was not what he would have hoped to do, but where is he meant to draw the line? After the child attacks you (or a sibling) physically, or before?
You say the boy accused you of hitting him. And that it wasn't true. Stuff is going on in his head. That's such a classic. Look at women's refuge list of the most common statements. "I never touched her she must have fallen, she did it herself, she hit me first, I was defending myself" are top of the charts. "She hit me" was the police murderer today. He choked the woman to death, breaking three bones in her neck, then cut his own arm ready to say she attacked him. Then said he accidentally fell on her while piggy backing, then his hands accidentally somehow must have been squeezing her throat till the bones broke and eventually she died (she was still sitting in her car all this time) Then he left her without notifying anyone. And he got away with it.
Let's hope this grabbing by the lapels incident will have pulled things round, but I would suggest you stop abdicating discipline and leaving the two males yelling. That's a form of assault the family is being habituated to. If H wasn't there, you would simply be forced to face up to insufferable behaviour on your own, (and you have a trainee mother batterer and wife batterer in the making, because much as you love him, that boy very much needs strong boundaries. If ever the phrase 'tough love' was appropriate, it's for re-training an aggressive teenage boy to moderate his worst behaviour)
Therefore, maybe start right now, form a united unbreakable front with H, after privately agreeing new tactics with H. He won't yell or threaten or grab, as long as you pull your weight and never walk away or ignore anything. Both of you, together, must firmly tell awkward S every time he steps out of line.
You want H to say "Don't dare speak to your mother like that" ("your sister, your brother like that") You must stand up in the same way for H. "Don't dare speak to your father like that" and "Don't dare raise your voice in this house".
(As concession for his personality, let him get away with flouncing off and slamming the door, rather than backing down or apologising. And never mention it afterwards, because as you say, he will never be wrong, in his own eyes. It's enough that he becomes re-trained to stop the unacceptable behaviour as soon as it is challenged )