I'm so sorry OP, its really difficult. Its one of the reasons we have no savings in DDs name, but that is based on seeing outcomes very similar to this, its definitely not how most people I know do it.
In terms of the weed, I hate how much it has been normalised, or treated as if its only a problem if its a 'gateway' drug. I know two men who developed drug-induced schizophrenia at a similar age, I'm afraid to say both in their thirties are profoundly affected, one was never aggressive and so despite repeated mental health admissions was only ever a risk to himself, the other unfortunately was and still is extremely aggressive.
He has huge issues managing money, but again in between episodes of being committed he hits the threshold for having capacity. Its a huge burden for the extended family, there is a lot of worry, and very little to be done.
However, I do also know people who came back from heavy weed use at this age. I think for you, I'd start focusing on what you can do to protect yourself. He sounds like he is very abusive. If you did say he had to move out, what would happen? Would it get worse, would there be any help? Is there anything you can use to try and get him to access some form of support now, before he is 18? I think the best thing you can do for him, genuinely, is get supporting mechanisms in place for yourself. If the money is gone, the money is gone, but you need to start figuring out the best way to handle it - it might be that getting him to use the money as a deposit somewhere else could help. Are there any other adults he might listen to? There are al anon equivalents for family of drug addicts, I think now is the time to start reaching out for yourself.