Different people react in different ways. I didn't get stalked or almost murdered, but I was pretty paranoid by the time I got to 17.
PTSD is a good response, as is 'always on guard' and 'carrying a weapon'. He might well try to blend in with the crowd on the surface, while knowing that underneath it all he's not like the rest of them. That would mean finding reasons for having unobtrusive weapons like steel-toed trainers/sneakers. I took an interest in theatre tech work at that age, and in those days steelies and a folding knife were excused by the world at large as tools of that trade, or at least not looked at too oddly.
He'll know where the exits are from any room he's in. He'll have an exit strategy for most buildings and most situations, and he'll know which parts of town are safe to be in after sunset. He'll know how to draw attention, mostly by knowing how to divert it. He may well know all sorts of bits and pieces about anatomy - an idea of how to defend himself against anything - and all sorts of things about weapons. He'll probably know the little things that most people never think of, too, like not closing the curtains with the light on, or how to move silently on almost any surface, or how to vanish into the vegetation.
Life could become a constant training regime so that it never happened again, so that if it ever did he'd be not only prepared but able to do something about it. It could get to where reflexes were rewired so that if X happened, Z, A and B happen instead of Y.
Those are the paths I took with less cause; others would, will and have taken other paths.