Me again.
Boisdeslandes, you can only help someone who wants to accept help, you can't help someone who won't help themselves. There's no formula or method to force help or change on someone who doesn't want it.
What you could say to your son is what we said to S recently. Tell him that there's nothing you can do to help him if he's going to continue in his old ways and mix with his old drug friends. However, if he suddenly decides to change, then you will be there to help him and give him all the support he needs. That way, you're laying the decision on him. You are saying that you will help him and won't turn your back, but it's going to be him that needs to make the decision to change and have nothing more to do with drugs or the drug crowd.
All the best.