OP, affairs are rarely black and white, and it's true to say that many affairs are born out of difficulties within the marriage. However, in that instance it's also not fair to say that he was responsible for the breakup etc, if his behaviour was such that you felt you had no choice but to leave, but had an exit affair, then the truth is that you are both responsible for the breakdown of the relationship. And regardless of his part, he has every right to feel bitter and to want you to leave the house now that you have made your intentions to leave him clear.
As for introducing your DS to the OM while you are still living with your DP, no. Absolutely no way is this acceptable - ever. It's IMO not acceptable to introduce new partners while you're living with the other parent even if they're not your affair partner. You need to move out first, allow time for your DS to settle into wherever you live, and only then do you even start to think about it. A year at the very least given the circumstances of this relationship. And no, the fact you know him already is irrelevant here. Your DS doesn't* know him, and that is all that matters.
You were deluding yourself if you thought you could just leave your current partner and introduce you and your DS into the new life you have planned with the OM. There is going to be much unravelling to do. It will take years before you are even remotely in a position to be living a normal family life with this bloke, if ever.
As for having just picked up where you left off, don't be so bloody naive. I mean where you left off was the end of the relationship, no? And in the interim you had another serious relationship and a child, and presumably he has lived his own life in that time. Why did you split up the first time anyway?
Sorry OP but if you care about your DS and want to do the right thing as a parent, then you will put him first and put the happy ever after on hold for the foreseeable future. Keep dating this man if you want, see where things go in the next several months to a year, and then, and only then, think about how to introduce him to your DS.