Jake, you know my thoughts on this, but I'll reiterate - the self medicating and drug taking etc is a choice that he's making - whether it's informed or not, it's still something that he's doing. Ultimately, it has to be his choice to "get clean", or "better". You can't force him into rehab or similar unfortunately - all it will do is create a feeling of contempt or "me vs them".
Like with my own situation that I've told you about, neither you nor your parents are professionals - whilst you can act with his best interests at heart, ultimately you need some professional help or guidance. I'd suggest you and your parents speak to a professional about how you can interact with, and help him. In addition to this, unless his has shown that he is a risk to other or himself (through police/hospital/specialist reports), then it'll be very hard to have him detained under Section 4 of the Mental Health Act - unless a GP or similar is in agreement with you. I'd guess that the second that your brother saw one of those, his behavior would somehow improve and he'll be "perfectly fine" and deny everything. Then it's your word against his.
Fingers crossed for you.