Welcome! Sorry you find your self here, but under the circumstances there's no better place to be.
I know you want to help, but this doesn't sound like something you can fix. If he won't seek help ther's not much you can do. Let me ask you this, how do you think he will respond to your talking to you BIL and Dr? Do you think he would be more recptive if they intervened? Keep in mind at some point it will probably come out that you approaced them. This usually doesn't go over well.
I agree more info would help, but at least this bump will put you back up top for others to respond to.
