Ok this is just a suggestion...so only take it as that, because this is a very delicate matter as you well know.

If you brother needs help then maybe going to the police will be exactly what he needs. Often in these cases they will make him go to a rehab, court ordered. It may not really cure him, but maybe it will be a wakeup call.

You could say oyu don't know who took them, but the police would know, and so would your doctor. They would probably investigate your brother anyhow, because these kind of crimes occur by someone who knows the person. The hard fact is that your brother is already in trouble and eventually it's going to catch up with him. Reporting him will won't make his condition worse...it will Best if kept off the board him off obviously, but it could help...

But honestly, I don't know what I would do, because it's your brother and he's family. It's hard. However, like I said this could help him in the long run...letting his habit go to those extremes isn't helping him either...
Good luck, my thoughts are with you.