another baby

This isn't one of these post about myself having a baby. Here's some history before I get into this. I have a two year old he'll be three august 2. His dad and I have been off and on for years. Right after our son turned one we decided to separate. We are really good friends and co parent well. A month after we split he has been off and on with the girl he was with before we got pregnant. I honestly can't stand her, I'm nice to her when I see her. They moved in together in July. I just found out they are pregnant. They are due 2 days before our son's birthday. I want to be there for him without being emotional because I do really still care about him. I need advice on how to stay detached emotionally and still be supportive.