Living with depression, autism, and ADHD

How can one encourage someone to seek help when they are unwilling? He is now 18, which means I cannot force him to do anything. He shows no interest in activities and rarely communicates verbally, responding only with nods. He has no friends, and this behavior has persisted for the past three years. After giving him some space by moving away with other family members, hoping things would improve, he still denies having any mental health issues or suicidal thoughts, despite being diagnosed with ADHD at a young age. As a child, he was happy and outgoing, interested in sports and very funny. He does take care of basic needs like eating and showering, but when I try to take him to see a psychologist, he refuses, claiming there are no problems that require treatment or counseling. It is heartbreaking to watch someone waste their youth like this. Any advice would be appreciated.