S.P.
My son did the same thing when he was that age. Just having someone talk about fire safty doesn't always drive the message home. I went to the fire house and had the fire chief talk to my son. He did so very sternly and then showed him a movie of a house started on fire from someone lighting matches and one of a house on fire because of Christmas lights. At first, I thought it would only scare my son but he never played with fire after that. We also talked about burns and how bad they can be. It seemed to help. If one of your friends is a firefighter they should have some tips for you too. Also, since it happened at your in-laws maybe he was acting out because something upset him. Try to bring up the subject sometime when he's not in trouble. You can get better results then. Good luck!!!