another vote for the grudge though! if you're really sadistic, download that noise from the grudge, put on mp3 player and hook up to a mini battery operated speaker with 15/30minute interval's and hide it under his bed haha. i might try this myself.
Been there, done that. My female housemate in uni had a love/hate relationship with that film, she liked being scared but was terrified by it at the same time.
One evening when she was out, my other housemate and I set the desktop background on her laptop to a creepy picture of the little boy from the film, and set the grudge noise as the Windows startup sound. She came back later that evening, went into her room in the dark, turned on her laptop, and all I could hear was screaming whilst I was lying on the floor in the hallway crying with laughter.
A year later after we watched the second one, I put the sound on my phone, set it as the ringtone, chucked it under her bed and called it about half an hour after she went to bed.