Thanks everyone for your replies. I know I can't control him I am mostly upset that he continues to do the opposite of what we've agreed on. Nothing is in writing yet - he hasn't seen the first draft of the decree. Everything we have agreed on is been through conversation and/or emails.
It's hard enough for me as the BS to deal with the fact that he's with someone else but now I have to worry about him exposing my son to her so soon. I told him this isn't forever but right now it's too soon. He agreed.
Initially I was going to stay in our marital home but decided it was too large for just my son and I and the mortgage too much for just me. Plus he said if we sell it then he can afford a nice down payment on a new home so we can both have a home with a bedroom for our son. But now that he has moved into an apt. with her where does that leave my son? My son has been sleeping with me through all of this. He goes to sleep in his bed and always ends up coming to mine.
To keep my son's life as normal as possible my STBXH comes to stay with my son in our marital home on his weekends and I go stay with friends or family. It's worked out great because our son has his friends nearby and his toys, etc. and he feels comfortable at home as any child would. I've talked to my son at length about how he and I will get a new home and he'll get a new room and how exciting it will be and I tell him the same about his dad's new home. Now I don't know how to prepare him. I can't even begin to imagine him waking up and crawling into his dad's bed with him and her. *sigh*
I would never request full custody where my STBXH gets no time with my son. I am not trying to take him away from him. I would never do that. My son needs his father. I would never stand in the way of their relationship, ever. The only thing I have asked of him is to please not have any overnights on his weekends with our son and not to introduce him to anyone new yet. The same goes for me. I'm held to the same because my belief that it's damaging to my son goes both ways. We are both responsible for his emotional health. Again, this is not forever but for now this is what we agreed is best. Of course he doesn't know I know that he has moved in with her so he must be blowing smoke up my a$$ for the time being.
I was so upset yesterday when I discovered they moved in together because where does that leave the whole no overnights deal? In the toilet I guess. I am really disappointed in my STBXH and his lack of respect (clearly he has none for me or he wouldn't have had an affair in the first place, I know) for my view on parenting. If we are doing this together we each have to respect the others wishes. Clearly that's not happening so I do need to prepare myself for the inevitable I guess.