I am going to share my story with you. The one thing I will say is you cannot push, push, push to force him into seeking help. My story:

I have an adult son (26) who has ASD, ADHD, anxiety and depression (diagnosed at age 10). He has been in a spiral for about 6 years now, do to a very traumatic event that had a negative impact on us both. I have gently suggested he go see somebody for help. His response was he doesn't want to be on medications as they don't let him feel. Recently, things have gotten more mentally stressful for both of us. I have been seeing somebody and am on meds. I suggested, about 2 weeks ago, he go see somebody. I brought the intake forms home with me. He finally filled them out acknowledging he needs to do something.