I agree with Winterybella, you should talk to him about the marijuana use and why he is doing it. It will show your son that you are the kind of parent who cares what is going on in his life. There are so many parents that go into denial about such things. Perhaps there is a reason that he feels he needs to turn to marijuana...stress in school, peer pressure, ect. Kids at that age are very impressionable and maybe he just feels that it is the "cool" thing to do and might not even realize how bad it is for his health.