D.K.
Your feelings are so very normal. I couldn't stand my ex but had the same feelings when he started dating (he is on his fifth girlfriend now!). I think there is a huge loss there and you haven't moved on yet so it is very hard to see if it didn't work out with you how he can all of a sudden find someone to get along with and be happy with.
It takes time, a long time. I think it took me a good year or so before I could care less who he dated anymore. I just felt sorry for the woman knowing his attitudes, mood swings and issues we had. My ex is thinking of remarrying, which is fine, if he is happy then my kids will benefit.
I would however make a very firm stance that unless he is really serious with this woman that he will not bring women around your son. That causes a lot of confusion and hurt for a child having rotation in adults they get close to!!!
That rule should apply for both of you.
If you have shared custody then he can wait to see her when he is with you and not be selfish with his time. He should be devoting his time with his son anyway.
If this relationship is really serious then you should meet her so you know who is in your son's life. Your son will never love anyone like he loves you so don't worry there! The more love your son witnesses and peace with everything he will grow up a lot happier.