When you ask him just what in the Hell he's doing and he says, "Getting some fresh air," you can
  1. Kick his butt out the window
  2. Tattle on him to his father
  3. Laugh
  4. Cry
  5. Yell about your worry over his health, your concern about his falling out the window, your fear that he'll set the house on fire, your discomfort about what the neighbors might see (head out the window, plumes of smoke)
  6. Talk to the butt (since his head is still out the window) about limits and how he's not to see so-and-so anymore because he's just a bad influence
  7. Stand there gawking and wondering if it's pot not cigarettes because you are getting the distinct smell of weed
  8. Walk away, sighing, and hope for the best
  9. I chose walk away, sighing, and hope for the best
  10. Leave the house immediately to grab a drink with another parent-of-teenager
    Suggested by @laurimontclair