Hello my friend. I am sorry to hear about the relationship with your son. One thing that you have on your side is time which does heal all wounds. Keep the emails coming, the phone calls as well as the letters. That way he knows that you are still interested in his well being and that you care and love and support him. He is probably going through some tough times emotional, professional, and financial. He may be too embarassed to come to you for advice. If you have no idea what the source of the conflict is between yourself and him, maybe you can have one of your other sons casually ask him by saying, "Hey, have you talked to mom lately? Why not?" Then that son should let it go and tell you about your estranged son's response so that you will have some ideas about why he feels the way he does.
The main thing to understand is that we cannot control other people and make them change. They will change when they are ready. We can only change our reactions to their actions.
Hope this has provided some insight. If you need more advice, please ask. Thank you.