BortimusPrime:

He should just go clean his parents' house while they're away, and make sure to disturb all the untoward things he finds (sex paraphernalia, severed Korean heads from the war, etc.)

Well since it's more likely that their house is spotless, he should go there and mess it up a little bit. Then tell them that he wanted to make their house look like someone was actually living there, because robbers might not think anyone is there if nothing is sitting out, and because no one could live comfortably if they have to worry constantly about messing up the house, so he was just doing them a favor. Oh, and while he's there he should be sure to throw something of theirs away and then leave some condoms on their kitchen counter.