How far along the route has he gone ?
Does he have any idea of what trade he wants to follow..................or does he just want to have a gun and go and shoot people ?
The Army Careers Offices can explain to him the type of 'trades' available. Quite a lot of which could provide him with relative safety..............and his mum with a better peace of mind.
I had the same thing with my precious boy, at 17. I told him that I wouldn't sign his papers unless he went in and qualified in things he could carry out to civvy street - which ruled out guns and tanks :o)
He's been in for 6 years. Yes, he did go out to Afghanistan. And apparently I needn't have worried half as much as I did.............and I will feel better should he have to go out again.
Reverse psychology may work on your boy..........become really enthusiatic about the Army, choose a new colour-scheme for his bedroom for when he moves out......
Other than that, breathe deeply and tell yourself that whilst you are not neurotic, you are not a wide-eyed 17 year-old boy, either.
Best of luck.