Ok. There are several things at play.
You cannot change him. You know that. You cannot get him services that don’t exist, or that he won’t accept. You have tried everything. He is not going to suddenly understand.
You have to remember the impact of his communication issues, even though you know him really well and think you are communicating well with him, you probably aren’t.
He sounds as though he knows he is right, and therefore everyone else is wrong. That makes him indifferent to advice or coaching.
All you can do is manage your own situation, where it is impacted by him.
When He asks if he can borrow money, you too can be like Ariston. Just say no. Don’t explain. Just say no. If he asks why not, ‘I haven’t got enough.’
Only work with him, or make suggestions, when he actually asks for it. He may need to feel the failure to be ready to ask how to avoid it.
If that means he gets ill, then so be it. It won’t be your fault, it will be a consequence of his condition.
All you can do is manage your own situation, where it is impacted by him, and help him appropriately when he asks for it- not give him money, but help him access resources or learn new skills when he asks for it.
He is an adult. He is making choices, albeit unwise ones that he doesn’t fully appreciate.