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.
“World War III is a guerrilla information war with no division between military and civilian participation.” — Marshall McLuhan, from Culture Is Our Business, 1970