I have had the exact same problem.....tell him to clean them off himself, check the car when it returns and hand him the wax.....I was always taught when you borrow something, return it as it was.....after making my son clean the dirty Viper/Prowler etc a few times, he found it wasn't so much fun to drive it so often....once, he stood and waited for me to finish cleaning it, then took it for a long day out and left it covered with bugs and dirt for me the next day....thats when the waxing detail first came into play.....also I have control of the keys....no real answer till they leave home...mine is 25 and still "LIVIN FREE".....