That is the fundamental problem right there. His conditions need treatment. What you’re seeing now is what you get with untreated mental disorders: inability to keep a job, self medicating with street drugs and alcohol, poor relationships, etc.
Think about how you would respond if he had cancer and refused treatment. Whatever that looks like, treat this situation the same. While ADHD is not a terminal condition, untreated it dramatically reduces one’s quality of life and greatly increases the chance of premature death.