How old is SS?
Does SS live with you full-time, part-time (50/50) or at weekends/holidays?
Do you have DC with DP? ... or is it just you have DC's and he has DC's?
If your DC's get on with DP and his DC's, then DP moving out shouldn't be an option. (It will upset lots of people, including you).
As others on here have said, yes, you should detach yourself from SS for a bit ... don't make him meals, etc.
I 'detach' myself from my DP's son at times, and it seems to ease tensions in the house. Me and DP's son tolerate each other, up to a point, which obviously upsets DP, but deep down DP knows me and his son will never be the best of friends, plus I have to take his sons age into consideration (DP's son is only 7).
Similar to what chickensaresafehere said ... I have tried to get on with and form a relationship with his son for 2 -3 years. We get so far 'bonding' then DP's son decides to 'go off on one' at me and things fall apart again, so as I said before, we tolerate each other.
Even now, I know his son will be arriving with DP, at this house for the weekend in approximately 4 or 5 hours time and I'm feeling sick to the bottom of my stomach ... but I will put on my happy face and smile through it, for the sake of his child, in the hope we will have a decent enough weekend ... some weekends are lovely, others are hell on earth, but they balance each other out with a half decent one every now and again.
Hope you can get this sorted without too much upset