I really feel for you, i caught my 15 year old daughter with it, too. I took her to the GP, who spoke to her about the risks of it and she was referred to CAMHS (google for one near to you). They are for children and adolescent mental health and are really helpful - but you do need a GP referral. My daughter has since been diagnosed with ADHD and apparently drink and drugs are quite a common way for them to self-medicate. There may well be something else going on with your son, that he is trying to treat himself. My daughter is seeing someone from Addaction and they have suggested Frank, if needed. She is due to start low medication soon and has been put on different courses, too.
There is help out there - he will need to co-operate for you to really get any results, but even if he says he won't, they will still guide and advise you.
good luck xx