To put things in perspective, here in the US a parent might have a similar reaction to catching their kid drunk. Yet you have probably been giving your kid wine with dinner every night, right? (Based on the little I know of France.)

Most of the problems with alcohol, and marijuana, are really caused by societal disapproval of them. In the US a kid can get in a lot of trouble for either (though much more for pot) while actually using alcohol or pot has little or no harmful effects.

So, definitely don't call the cops. That could do far more harm to his future than the actual marijuana.

This is a very pro-marijuana website, so I'd say do some research here as well as elsewhere to find out what the real risks of pot are. Then talk to your son, make sure he at least knows it's illegal and getting caught by police could cause serious problems. Other than that, you'll need to form your own opinion as to whether or not it's really OK.