Why do people spend so much time feeling bad about themselves. Living a long way from your children and grandchildren is quite a common situation, as is not having cousins, in these days of smaller families. These problems arise frequently. They are situations millions of families face.
The thing to do is look for solutions. Talk to your son, ask him whether he has any suggestions. In the posts above mine people have suggested lots of different situations, which you can discuss with him.
Situations like this are caused because people sit around 'feeling bad' instead of standing up and finding solutions, of which the first step is to talk to the other people involved about the difficulty.