This is a tough one! You must be worried sick! Was this a "final straw" moment, because it seems telling him he should leave if he doesn't like the rules was too big a response for what was happening. Leaving isn't an option, and that's something you will need to address (and possibly apologize to him for) when he comes home. He's probably very hurt right now, as you are. This may actually work positively for your family to start a conversation about his behavior and your concerns.
What other consequences does he get for not following the rules? What things are taken away? Have you clearly explained WHY you don't want him smoking (health risks, etc.) not just that it's "against the rules?"
Since I don't know what your relationship with your son was before this all started, I don't know specifically what to do to help you, but usually if they change their friendship group or behavior, then something ELSE changed in their lives. Some conflict at school or home. Yes, teenagers will push the boundaries, but that's their job. You as the parent need to preserve the boundaries because that makes them feel safe and gives them guidelines.