My brother-in-law did quite a time in the army and was a hard man. He got invalided out (duff knees, not enemy action) and still misses the life and the comradeship that the army offers.
I realise you're going to miss your son and worry about him. Most mums would. Just accept that he's doing what he wants to do and be proud you seem to have raised a fine young man.
As others more knowledgable than me have said, send those letters and send those prezzies!
The best of luck to you and your family.