@OverTheHammer I am about 9 months down the road from a similar situation with my DS, 16.
Being stuck between DS, DH and other DC is so so heartbreaking and knowing where to start is incredibly hard, as you love everyone in this situation and do not want to sacrifice any of the respective relationships with each individual.
Apologies,in advance, if my post comes across as patronising at any point or a repeat of any PP but this is my take on it:
I can understand why PP's think calling the police is the right thing to do, as essentially assaulting someone is a criminal act... however he is still your child and children are not 'born bad' and as parents we must take responsibility for our children's behaviour especially when they are still minors.
I understand your DHs fury but with all due respect, he 'signed up' to help parent your DC or he should have married a woman with no children. Sorry if that sounds harsh - I know being a step parent can often seem like a thankless but your DH has to realise that squaring up to an already agitated 16 year old is unlikely to ever result in DS seeing the error of his ways.
In my experience, the situation will never be resolved in the heat of the moment and what is clear is that your DS does not seem to have a male role model who doesn't use physical force as a way to 'resolve' issues (This was the same with my DS). Your DH needs to part of the solution not part of the problem.
As @MissJC suggested, can you try talking to your DS again (just you two first)? As well as outlining what you find unacceptable about his behaviour remind him of any recent examples of his behaviour which shows he is a 'good person' and also give him the chance to voice what his frustrations are, as well as anything that is happening in the household that he would like to change.
Discuss his frustrations fully with him and never dismiss them totally out of hand, even if they seem irrational to you. I am not suggesting that you bow to his demands but if your DS can see your are willing to explore his perception of the situation you might have a better chance of a positive conversation and outcome.
(Eg I hear you feel that we are always on your back about playing the Xbox, but we are not asking you to stop playing it completely but just to understand how unfair it is to be woken at 2am in the morning when you need to wake up 4 hours later for work. Let's agree a cut off time, so that this is not an issue again and everyone knows where they stand. What time do you think is a reasonable time to turn off the Xbox at night...?)
Agree general do's and dont's - our children are never too old for boundary setting.
Give him the chance to hear your (and DH's) alternative view, in a calm manner, and agree what would be the best way to resolve issues in future when they occur instead of just telling him how wrong he is, in the heat of the moment. And underline that physical force on either side is a big no- no.
That way when unacceptable behaviour does occur, you can just refer to your conversation and remind him what was discussed and agreed.
Underneath that cocky, bad tempered young man is probably a scared little boy who just wants love and direction.
You are not a bad parent, each of our children have different needs and what works for one child does not necessarily work for the another. Children who cause the most problems and unpleasant circumstances are usually crying out for love and direction, in a different way.
I really wish you the best. xx