Ex-husband starting new relationship while his old girlfriend is still living in his home?

My ex and I have been divorced for several years. I have been in a stable relationship with the same man for the last 4 years. My ex has been with his girlfriend for 2 yrs. Suddenly he has decided he no longer wants to be with her and has taken up with some childless bar s**t. The girlfriend he just dumped refuses to move out of the condo they share since she has her own little boy to think of and has no place to go. This woman by the way has been a wonderful stepmom to our son and we plan to continue a friendship. My ex is pushing that I let this new woman meet my son but I said not for at least 6 months and not until his old girlfriend is out of the house. I don't want our son, who is only 5, to be horribly confused and upset by all this. He has also started going out & drinking every weekend - weekends that he's supposed to be with our son as he has him on the weekends. I know this must sound crazy but any advice would be greatly appreciated.