OP, it seems that your DS tried to shove and hit his father soon after your split. Since you were not there for the argument, it is hearsay from your teenage DS.
Is there a third party that you can approach to help you with this situation? A mature family friend who can talk you your ex-H and let him know that while he may have an issue with his child physically assaulting him, he is still the parent and the adult.
The trivial issue that triggered this latest spat is likely not the underlying cause of all the anger leading to physical blows. Six weeks is more than enough time to cool down and reflect on what led up to the physical fight and the fight may have been percolating for a long time since as you've said, you and the kids walked on eggshells prior to your ex moving out.
As for DS, you need to give him support, but firmly state that it is not right to lay his hands on anyone. If he is upset with his father, he can always walk away to another room.
In addition, and this is pure supposition, you need to also be firm that DS does NOT need to fight your battles. DS should know that it is not his job to be an avenging angel for situations he may have witnessed growing up in a tense situation, as you have described. DS may need some professional help to work through his anger issues, so he does not get into physical altercations where he could be hurt, stabbed or worse.
It is a tricky situation. Good luck OP and hope your family heals and you can get to a better co-parenting situation.