UpAllNIght, if your son signed up for the Air Force or the Navy he will most likely NEVER see combat. My husband signed up with the Air Force at age seventeen to help his mother pay the house mortgage after his father died. He saw the world, but never saw Nam. He did work on airplanes and his hearing is impaired, but that's all. He served for ten years, until his first wife begged him to quit. He quit and she left him. OH, well. I think he would have been Career Military otherwise. He wanted to fight, being a silly seventeen year old, but they wanted him as a mechanic so he was a mechanic.
Now if your son joined the Army or Marines, they do go into combat. What did he sign up for? No matter what, they may want him for something other than combat.
I understand your fears. I really didn't want any of my kids to choose the military, even though my husband is a vet. But it IS a very noble thing to do. He is a brave young man.