Gonegrey, I am sorry you are going through this. Does your DS have a part-time job or does he completely rely on some allowance from you?
Is his Dad or a Dad like figure on the scene, or is there some other male role model that he might listen to. I think that every teenager thinks their mum is either a pushover or mad so it makes it difficult to help them. Does he have a sports hero or someone like that that you could maybe write to and ask them to talk about drugs with him - a long shot but not unheard of.
I bet the police have got some information to help parents in this situation. Is he still in school? Maybe the school counsellors can help.
It would be so good if he could have an 'ah-hah' moment now rather than later. Sadly such ah-hah moments often consist of seeing a friend badly injured or OD'd or busted by the police or whatever, or missing out on something else he really wants - like a girlfriend or a certain mark in school.
Does he have other interests or ambitions that you could encourage him to channel his energy and time into (you know the old use the positive instead of the negative - instead of don't smoke drugs - why don't you join a football club, or buy him a PS2 to play at home or I don't know what else).
My FIL bribed my DH not to smoke cigarettes by promising him he would buy him a car when he turned 18 (nothing flash but ...). I know bribing is not in vogue, but it might be worth a try.
I know 15 is pretty hard to say you are grounded and can't go out or whatever but you do still have control over him formally. What about rehab for teenagers - surely there would be camps or something.
Anyway as you can tell this is just a stream of conciousness for me, but I hope that it gives you some ideas. Don't worry about being naive or anything. I am sure that by the time my DS is a teenager I will have no idea about what is going on.
Good Luck!