It is accepted that some people are particularly vulnerable to becoming addicts of substances/chemicals, be they "drugs", alcohol, nicotine, etc. Such people lack sufficient self discipline to halt or even curb their use of whichever stuff it is. Moreover, while some would like to stop their substance abuse yet fail to others actually become defensive/angry when it is suggested that abuse is taking place and their reaction is even worse if they are encouraged to reduce/stop. I have personal experience of this as someone who has been "excommunicated" by an addict (alcohol) who is a close relative.
The simple fact is that it is not possible to do much more than express an opinion and accept the outcome where an adult is involved. Unfortunately for you, drug addiction is far and away the worst case, it can be devastatingly destructive, robbing the user of all judgement other than in connection with how to acquire the next fix. Apparently distinctions between right and wrong can disappear entirely. Society is largely at a loss of how to deal with the threat and the effects following its arrival but some schemes are emerging giving some hope (mostly outside the UK).
In the end it boils down to people being willing to give the lifestyle up. I happen to know someone who finally decided she could not continue such a life - she was a heroin user. She went for rehab (for the nth time) and more than 30 years on she has stayed clean. Crucially, it was her decision - her friends and family had previously tried persuasion and her parents had repeatedly sent her for rehab but she kept lapsing - until that decision. She didn't complete the course that time either but stayed clean nevertheless.
Worryingly, all sorts of calamity can ensue in the course of addiction, such as infection. My friend had Hepatitis C (non-infectious carrier) and recently shed the infection completely with the latest drugs. The most terrifying possibilities are those of overdose and contamination which both are closely related to sellers' greed, incompetence, etc. and the addict's relentlessly furious appetite.
Your concern is entirely understandable but it is important to see things realistically - the situation is almost certainly utterly out of your control. How much help is there in worrying, however natural that is ? The best you can do is hang onto hope.
Hopefully your son will in time emerge from this. He will never be his old self again but remains of himself will be there. My friend has a number of physical and psychological complications resulting from her life of addiction but she is very obviously a good person.