If the op moves her new man in, she'll lose her son on so many different levels. I have a friend who moved in a new man when her boy was in his early teens and though he stayed at home, he has increasingly become distanced from his familial home. Now he's 16 and he spends no holidays with his half brother and his mum. He either stays with his dad or, more recently, his girlfriend.
He's a lovely boy. He loves his mum very much. He doesn't get on with his step dad. And lets face it, he doesn't have to!
He has told his mum he loves her and doesn't particularly want to move away, but feels he has little choice. He understands, however, that his mum needs a life of her own and that his adult life is about to begin, so he would never give her an ultimatum. Having said that he needs to be happy too.
What I'm trying to say, in a long winded way, is that you'll lose your son if you move your new partner in. If you're happy with that, go for it, but don't expect your son to play happy families for you. He doesn't have to if he doesn't want to.