I'm very sorry to hear this. When my elder son was 15, something similar happened to him. I found the best thing was to let him talk about it, which he did constantly for 3 solid days afterwards, and for me to listen objectively without too much comment. This 'working out' of it seemd to do the trick & he never spoke of it again & it has not ruined his life.
I am sending healing to your son, with much love & compassion.
Change the way you see things, and the things you see will change.