Some nice thoughtful replies on here. Hope they help a little bit
I've no great wisdom to impart, but I'd echo some of the comments & suggestions.
Main one would be speak to a drug charity, or service, they must have been through this very situation multiple times and can advise you without prejudice.
My own quick thoughts though.....
It'd be good if a man to man time could work as a poster suggests and maybe sharing stories of your own time smoking etc. Back in the day I used to smoke daily for a while and if it were possible I'd like to get over that smoking weed in general '''might' be OK but smoking it every day really isnt.
I don't know if any 16 year old is going to listen to stern advice or actions, or accept you saying it's a gateway drug etc. But they might get that if they are gonna smoke weed regardless of what you say then it should be a weekend thing, like it should be with booze etc.
That's just my take, but as above, speak to the professional services about it and go with what they say.
Good luck.