I think I'll say:
Yes, & I didn't enjoy it, despite persevering for years because my mates were stoners (I was more of a speed fan - might gloss over that a bit!).
I'd also say that I knew loads of people who took drugs when we were in our late teens - a few who had their lives spectacularly de-railed as a result, a majority who just don't bother any more, & a few who still have the occasional spliff. So OK, it probably doesn't do most people any lasting damage, but it's still risky.
& then I'd point out that a lot of the stuff around now is waaaay stronger than was available to me, & there's evidence that it does your long term mental health no good at all.
Oh, & I'd mention my mate who, at 25, had his dream job lined up in the States - & was deported after arrival at Miami Airport because an 8 year old conviction for possession came to light.
Also, it has social associations now amongst many teenagers which are negative. When I smoked my first joint, it was seen as a cool, rebellious, 'alternative' thing to do - these days, it's regarded with amused contempt by a lot of kids - definitely for 'losers'.
Overall, I wouldn't lie - the reality is that every teenager in the country is going to be offered cannabis at some point, & 'my mum says it's crap' is going to be just as effective a deterrent as 'my mum would be horrified, she's never touched the stuff' -
I would honestly suggest you ask ds to take a good look at the kids at his school who are very much into their weed - he'll know who they are - there'll be a group of them in year 11.
Are they the academic high flyers? Are they running the sports teams? School orchestra? Starting their own bands? (Yes, possibly, but are they any good?)
When was the last time he saw one of them do something for someone else?
I really, really dislike cannabis.
Never thought I'd say that 20 years ago, when I just thought it made people boring - these days I see it destroy so much potential in young people.