You certainly need to begin a dialogue with your boy about your feelings about pot smoking and take some action. I would not make this a fire-and-brimstone lecture or an attempt to shame him or threaten him with extreme punishment. You need to ask him questions about why he does it, how long he's been doing it, how many of his friends do it, etc. Let him tell you the story of how the pot smoking began and how it fits into his life. This discussion and the ones that must follow should focus on what's going on in his life that concerns him the most, what worries him, what gives him pleasure, his social life, etc. rather than just talking about why he's been illegally smoking pot.
Seek out drug programs in your area, as well as a talented therapist who deals with kids your son's age who have had drug problems. Please don't jump to the conclusion that your son is a drug addict and needs to put into an institution. Treat your discovery of his pot smoking as a call to you as a parent to seek the help of professionals, trusted family friends whom your son likes and respects, and any other adults who have had a positive influence in his life. The goal here is to understand why he's doing this and getting him the help that he needs to stop. The idea is not to make him feel like a bad kid who needs to be shamed and punished. Get the help you need from those sources that I mentioned and any others that you think may be helpful. Believe it or not, this may be a chance for you to develop a closer relationship with your son.