sandy....
i can relate to your son in many ways... when i was his age i too walked to a bridge, (freeway overpass) i would find comfort there, and i would go there over and over to get closer to actually jumping.
you could either hope and pray it will pass, or you can try to help him.
its a very touchy thing cuz you dont wanna set him off... maybe you better get him checked into a psych hospital... he may hate you for it, but at least he will have somewhere to direct that feeling instead of turning it in on himself.