I don't have any advice for you being that I am only 25 and a happily married mother of two very young boys. I am just now getting use to always being worried. I am having a hard time being away from my 5 year old that started kindergarten this year. Guess my worries may never go away.
I just wanted to say it is so nice to hear how much you care for your boys even though they are almost grown. My parents have never seemed to care for my sister and I. I reciently moved back to my home town after living out of state the past six years. My mom lives less than one mile from me and she hasn't visited me in almost a month. I go over there about once a week and she usually sends me to take my 96 year old grandmother to get their groceries.
Even though your son is 21 and loving his independance I'm sure. I know it is very comforting to him to know that you will always be there for him and when he feels lonely so far from home he will smile knowing his mama is thinking about him back home at that very moment. Then, he won't feel so lonely any more. I believe that is part of what maks a good mother to an adult child because that is how I want either one of my four parents to feel toward me.
I had to buy my dad dinner the other day to get him to stop buy and visit even though he was only a couple miles away from my house. Nobody ever asks how my boys are and they never get love, gifts or sleepovers with my parents. I just wish my children could have grandparents like I did. Then again, my boys have parents like I never did and my goal is to be a mom like you seem to be. It is hard to overcome being a child of my parents. No, my parents are not drug addicts, or criminals just middle class people that happen to be very, very, very selfish.