Soldato
Paranormal Activity would do it.. it is quite believable from the viewpoint of a 13 year old.
It will probably mess him up for a year or so.
It will probably mess him up for a year or so.
Whatever you watch, make sure its at night, no artificial lights on a large tv with a decent set of speakers (preferably with a sub woofer)
The scariness factor imho isn't all about the film itself but more so the environment your watching it in.
Stephen King's IT
His mum and dad's "Private Video".
Scare the bejesus out him so it will.
Secure him to the chair for extra counselling bonus.
I knew a guy at uni who once found a video of his parents involved in some poop related activities.
He got sectioned a few years back. Not sure whether one thing led to the other.