Hiya.. you are right.. if your son does not want the help, im afraid you are flogging a dead horse.. is he working?
My mum was the same as me so i know what you are going through, and i worry if my kids do the same. I guess you just have to keep making him aware, and be there for him. But hes young and still has his future ahead of him and can change it now still.
My sons distant friends all cocaine and weed and ket all on snapchat and insta now! This young generation now can get it so easy. If he wants to stop he needs to cut all his using friends off and get some new hobbies.