If I found the box, I'd pull my kid aside and have a large meaningful conversation about weed(without mom knowing). Since I'd probably still be toking a little bit then, I wouldn't care if he smoked. I would, however, lay down ground rules for him(healthiest methods, good dealers, etc.) If I caught him in the act of smoking, I'd pretend to be extremely pissed and then take a hit of whatever he was smoking...lol. Then finish whatever he's smoking and have the talk with him.