This sounds like a lot of people’s worst nightmare, which is why they are so keen on distancing themselves from any thought that this could happen to themselves and believe that it is something that can be controlled.
If I were transported into this situation I’d like to think that what has happened has passed, and there is nothing that can be done about previous actions and situations and focus on what I can do in this situation to make change.
It’s not likely to be quick, but working on my relationship with my son and building up mutual respect for each other. Spending time together to focus on positives and not talk about the drug taking.
Addiction is so complex and it’s not for a parent to come up with a cure for this- professional help is what is needed, if this even is addiction. Just like (most) parents make hideous teachers for their own children. A parent’s job is to be a parent, unconditional love and make the best choices you have in the moment for all your children.
Counselling for yourself (through work, nhs talking therapies, charities) and your other children (start with school) would also be top of my list.
All children and adults need boundaries, putting in clear no drugs in the house and not to be under the influence while at home is sensible/ essential to safeguard the other children. You can support him to have some place safe to stay if he has not met this criteria.
The last thing would be to do what you can to build his self esteem to encourage him to see that he is worth so much more and that his life has so much potential. Hopefully he can start to take on the information about the risks of drug taking which are long term and irreversible - if this is just experimentation the “scare tactics” may work- if it’s addiction it’s something much more complex.
Good luck, ignore the horrible comments - they are from bullies and their words show more about themselves and their need to bring down others in order to feel some form of superiority than they do about you and your individual experience.