Yes that's a tricky one - especially as I am sure their are others his age doing exactly the same thing, so he won't want to be the one left out of the crowd......
When we first came here I showed my teenage kids a film called Brokedown palace, which was actually more about drugs, but the fact that it was a true story and they got to see the conditions in a Thai jail - I think scared them suitably enough into never wanting to break the law here just incase....oh and they are now absolutely paranoid about their luggage if we go anywhere - but that's not such a bad thing!
There is another more sinister dimension (I am another of many here with a teenage son and all the attendant worries). The bigger danger is not that the son ends up in jail, but that he is victim of the Thai scam. Same with drugs. He gets caught in town, then you get the phone call. "Million baht or you get done." You can haggle because this is a gang and the police are involved, but it will cost you a lot - or you get shunted out of your job and out of the country. Believe it. It happens.