I offer this with great caution. They 'young people' in my family all use recreational drugs. Two are into ketamine. The others use coke, e and spice. The only one that doesn't is pregnant.
They're not that young any more, all in their thirties, and have been using since their late teens. Every one of them is incredibly successful: senior consultants at magic circle firms, a high-level financier, a tech star, and so on. They work out a lot, are fit & healthy and in settled relationships.
I didn't stop using until my early 40s, and similarly enjoyed a very successful, happy, healthy and busy life. One of my siblings still does at 60, and is still successful, healthy, etc.
You'd be astonished at the prevalence of class A drugs in daily life, not to mention weed which is almost ubiquitous. I happen to think it's a problem that needs to be aired, since our lives are being run by people who're out of their heads half the time: and that's the reason this conversation never happens.
The fact is, "everybody" does it. The reason you don't realise this is that almost "everybody" is aware of the risks and manages their drug use responsibly, much as most drinkers manage their alcohol intake.
If the assault on your son was influenced by drugs, it's far more likely the yobs who were impaired by substance abuse. This, regrettably, can happen to anyone anywhere.
I'm glad he wasn't badly injured.