Very very well put Bogeyface, and exactly what I was thinking too, but I dont think I could have put it quite so eloquently.
I feel very sorry for what must be a confused little boy. He sees his dad once a fortnight, and daddy now has a new girlfriend, almost before he gets a chance to even make any sort of relationship with her, she gets pregnant, and he gets shoved to one side if the Op has her way.
Sorry Op, but your post really does annoy me. This is a child, this is your partners child, and you cannot erase him from your partners life. When you make a committment to a man with a young child, you take on a certain amount of responsibility to that child too in the role of step-parent.
You really should have developed your relationship with the child before getting pregnant, although that is too late now.
I really do hope that you havent purposely got pregnant simply to try and take this man away from his son. I have seen women do that before, and it is not nice behaviour.
You are the adult and it is up to you to try to bond with this child, and you have to do it. That child deserves it. Do not try and exclude him once his sister is born.