Check this out, Stacy. You come home one afternoon and enter your home. In the living room, sitting on the sofa, is your youngest child (a son here) with a gentleman you've never seen before. You acknowledge your son. He responds and introduces the man sitting next to him. Innocent enough, yes?
That's what I came home to about 16 years ago. My son's words were, "Hi, Mom. This is "xxx," he's my recruiter. I've joined the navy."
Whoa! You could've knocked me over with a feather. I was, at once, horrified and proud. Mostly horrified. In time, I accepted his decision. It did take a while to get over the shock.
The best think I can recommend is that you support your son in his decision - whether you agree with it or not - he's probably a bit fearful of what's ahead, although he'll never let you know that.
Oh, BTW, his older brother was a marine during Desert Storm. You want nerves?
Been there. Done that. Hang on. There are positives to the entire experience.