I also thought the stats suggest drug use has massively fallen, although it may have changed to more 'legal high' type stuff.
Drugs are always easy to find if you know where to look.
Lots of my mates were very druggy (I was always too much of a control freak to do very much myself). My siblings did their share too, both did well at school and uni and careers afterwards. My view is that as long as kids have stimulation in their life other than drugs, and their lives are otherwise going OK, it's a non issue. Drugs don't necessarily lead to 'harder' ones - of course everyone who has tried harder drugs has smoked gear first, but most people who smoke gear never even try coke, crack or heroin and never want to.
My kids are too young for this to be an issue yet, and I will act on the expectation that they will probably try drugs, but tell them they don't have to if it worries them and, if it doesn't, encourage them to do so with as much care as possible because there are risks. I would, for example, warn them about mixing drugs, which killed a friend of mine. The right advice is better than trying to scare them off doing it at all. But countless people do take drugs without anything happening, and alcohol has its dangers too (not to mention being in a car, anywhere, ever).
I would also probably intervene if they started very young, like your friend's son, OP. I think for under 16s things like drugs or going out clubbing can really turn your head and get you into unhealthy situations. I think once kids are a bit older, one with any kind of ambition can put it into more perspective and understand that preparing for the future is more important that getting off your face.
But at the same time, I do understand people being anxious - it's OK for people to feel differently about it.