Or make smoking in that particular place uncomfortable.. maybe by installing some powerfull lights.
This sounds like pretty reasonable balance between passive and active aggression. Cheap spotlight aimed at his smoking spot. Hear a cough, turn light on. No more cough, turn light off.
If he asks, tell him it's sound-activated to keep away burglars. He'll probably be too stoned to figure out why that's BS.
I think lights is probably the best idea, it also can genuinely increase safety.
We already have quite sensitive garden/"burglar" lights and they are activated every time he paces about. Save for installing a giant motion-tracked floodlight, this wouldn't help. Hanging out of the window and firing a waterpistol every time he sparks up might make him go somewhere else. The idiot has just started driving lessons as well, so I hope he isn't getting high before driving less I will be calling the police.