I'm going to have to vote for "do nothing" as well. No good will come from it and I doubt the police will even intervene. Although, I do like the fire extinguisher idea.

But, if you insist on doing something, if the kid does live with his parents you could always go over and politely ask them if they wouldn't mind asking their son to stop smoking weed in his car and possibly do it indoors our in the back yard. Based off of their reaction I bet you'll know pretty quick wether they know he's doing it.