I'd love to know how people 'just stop him'...
I smoked weed at that age - not the way it seems many do though, not the 'roll out of bed, roll a joint, apathetic depressed does nothing else all day' type of weed smoker.
I smoked weed occasionally, socially. Generally small amounts then chill out chat etc, occasionally a bit of a 'get absolutely wasted and fall asleep listening to Astro Celt Sound System then wonder where a whole weekend went'.
I could (and still can) take it or leave it, if the setting was wrong, company wrong, just not in the mood (at the time of writing I have a bottle of cannabis oil that hasn't been touched in months).
That kind of casual use wouldn't concern me, it is comparable to the social drinker who occasionally gets totally smashed but can go weeks or months without touching a drop (rare tbh, most drinkers will drink much more regularly than that).
The weed smokers/takers who use on their own, use at the start of the day, use it daily - that is a massive concern and I would want to find out what sort of user he is (and is hanging around with).
Without nailing my feet to the floor and holding me hostage in the house, my parents would have had NO chance of stopping me. They tried grounding me - but as they came home from work two hours later than me at the earliest, I just invited people round, and as I provided the venue, they provided the weed so the lack of cash also achieved nothing! I didn't have stuff to take away (90s, low tech family!) so there was no scope there for punishment either.