I want you to know that your concerns are not accurate (I had a hard time finding the right word). Meaning, the things you are worried about will not come true. For one thing the bond with your first, will remain intact. Your family unity you have now will not change in a negative way, it will only grow. I'm also sensing some birthing issues/fears. You can let those go as well.
So what I am seeing, is that you can let go of your fears and concerns. They don't have to be your reality. However, you can certainly manifest them if you continue to think the way you are. I think having a 2nd child will actually be a wonderful thing for family.
Your son will not grow up without a father. It will only be a short time. There is someone else for you and him waiting in the wings. He won't come forward for a couple of years, b/c he doesn't want to pick up the pieces. That is not what this relationship will be based on.