Don,
My heart is with you. I do not have an answer for how to handle it. Maybe, trust. Trust that he will do all that he canto take care of himself and that those with him will help to watch out for each other.
Get worked up! Let it out now and tell him how much you love him and support him. That is all you can do.
I was there, both as the soldier who had to be separated from his family and as the uncle who wanted to kick my nephews butt for joining up. It broke my heart each time he was sent to Iraq. He is safely home now, but I was upset .mostly that I couldn't speak my heart to him at the time.
Talk now. That is the only good advice I can give.
Aardie.