You say he was old enough to know better. He needs a good lecture and a few extra chores. Playing a "trick" with machinery, eventhough it didn't do physcial harm - THIS time - is nothing to let slide. While the "lickings" some were talking about might work for some infractions, it seems a bit joking to me. He has to understand that what he did COULD have hurt someone - like someone said if he does it with other machinery. Not to mention he COULD have ruined your whole project!