I'm sorry you're having such a rough time with your son going off to college and the possible change in his relationship with your ex-husband. I think anyone in your shoes would be having some struggles with these issues. However, due to the extent of some of the anxiety you shared, I agree with the other posters about looking into some counseling or therapy. Hopefully, a counselor could give you some strategies on how to handle all the emotions you've been experiencing.
Also, I'm not sure if you're a reader or not, but have you looked into any of the resources out there on dealing with an empty nest? I know from working at Focus on the Family that there's been quite a few books written on this topic. Off the top of my head, I've heard good things about Barbara and Susan's Guide to the Empty Nest by Barbara Rainey and Susan Yates and Give Them Wings by Carol Kuykendall. They seem to cover some of the concerns you shared about the choices your son will be making in college and how to deal with his absence. So, maybe you can find them online or at the library? Just a thought.
Well, hang in there and know that someone out there is praying for you. ((Hugs))!