am just off to bed, but you need to think about how much power you give your DS. He now has had the 'power' (and more importantly therefore, the responsibility) of making your relationship with your DP change/stop.
He has succeeded in throwing the 'other man' out of the house.
He is a young child, yet because of how he has been brought up (with all of the too much - too young that has been discussed many times before on MN) he has all this terrible responsibility on his young shoulders. He needs to be a child, to have fun like a child. He has constantly been given too much information, too much stress, too much choice, too much everything.
And now he has had yet another change to the household to deal with. Get a grip and move your DP back in (how on earth are you affording separate accomodation anyway?) and apply some consistancy. Your ds is crying out for rules, boundaries and stability. For someone to take the load off his shoulders and take charge so he can be a child again.
You're setting up a terrible sibling relationship for them too, DS2 is presumably currently scared of DS1, DS1 is jealous of the arrival of DS2 and now DS1 has 'made' DS2's father leave!!
I mean this kindly - please start listening to all the help you have available from external sources.
In your posts you do come across as flippant, you always have done. You also come across as totally detached, and every suggestion given you just say you can't do/won't work etc.
If you don't want your ds to go on to commit crime, have mental health issues or, heaven forbid, commit suicide, then please start thinking before you act and really try to get help and act on it. Good luck.