If your oldest and her hubby love him, accept him for them if nothing else. What a wonderful gift they've given him, a family he may never have had. And it sounds like he's turning into a fine young man.
Having been adopted myself, I know the thoughts that run through my head occasionally about what I could have been had I never been adopted. Love isn't dictated by blood, but by the heart.
Good luck to you.
PS, we have the ROTC here in the states. Same type of program, but they're designated by AFROTC, NROTC etc for the different branches of service.
Quando omni flunkus, moritati