A very difficult situation for you but my advice is to keep him home, don't let him go out unless you are absolutly certain you know where he will be and what he will be doing.
He may be bigger than you but hopefully he does have respect and is not violent.
The best way to keep him home is to remove his clothing or change it to something you know he will not wear. Keep calm and try to calm him down. Fluffy slippers can work wonders, so can a pinafore. Get him to help with the dishes etc.
You know his activities must stop, he has got to learn or as you say it will be the hard way in prison.

hi, ive just come across this website by chance. through mu own experiences if you dont nip this in the bud now then it may develope further. get straight to facts about what he is doing, dont beet around the bush. if it helps get in tough with drug support people who would come and talk to him. take him to homeless shelters and tell him this is where he will end up if he dosnt see what may become of him. im not saying that this will happen but a short, sharp shock may do him the world of good. its no joke if you feel your son may be going down the wrong path. please go and seek help and advice from people who can give you sum guidence, the poloce, ex-drug users ect. good luck and stay strong and positive