Aside from surveillance devices (good idea), perhaps a recent list of movies/shows/books he's seen or read? I know my son when he was between 10-14 had a super active imagination that drove me nearly insane. Not saying yours hasn't heard things but often helps to rule out (or in) the simple possibilities.
"It is hard to imagine a more stupid or more dangerous way of making decisions than by putting those decisions in the hands of people who pay no price for being wrong." – Thomas Sowell