I grew up in a blended family and have my own children as well as a step daughter. So from my experience I would say. You need to talk to his father. Stay calm and don't point fingers. Just explain that you want them to have a good relationship and that your son needs that relationship. Maybe he can make an effort with your son to spend some time just the two of them doing anything. Kids are sensitive to changes with parents and new kids. He just needs to know that no matter what his father loves him and wants to be with him. That the new baby does not mean his father loves him less, it just means he has a new sibling and someone else who when they get older will love him too.