It sounds like you are suffering from empty nest syndrome. My parents got a dog when we all moved out I think to help with those feelings, especially my mom. Try to feel proud of yourself for all the work you put in and that you son is successful and not needing to move in with you. In today's economy that is a major step. He is doing what you were trying to teach him to do as he grew up.

It is hard to feel like your son is abandoning you. Have you shared your feelings with him that you want to have more contact? If not, then he can't know that. Do you have a spouse? If so, have you talked to your spouse about this? Does your spouse have more contact with your son? My understanding is that overall guys are less chatty. This might mean that he is okay with the amount you guys talk and unless you let him know you want more contact he wont think of that. But I know I'm stereotyping with that comment.