I am a 25 year old female and when I was 19, I went through the same things that it sounds like your son is going through. I don't know if this helps or not...but I hope maybe my story can help you...
Anyways, I had just started college, and my ADHD was masked until then because I have a high IQ and the structure masked it. Freshman year of college I started not going to class, sleeping the day away, and only getting out of bed to shower, eat, use the bathroom, and drink. I then got caught stealing. (After 5 years of diverson it was finally expunged). That was the breaking point for my parents, as I'm sure it would be for any parent. I was using alcohol to self medicate. At first the doctors started me on anti-depressants and anti-psychotics so then I slept a lot too and was still very moody. I fought with my family a lot. And looking back, I wish I could change things for them. But, they had to be tough. I remember the bad days out weighed the good days. And there were days they had to just leave me be. BUt I remember the turning point was when I accepted that I had ADHD, admitted that I was scared, and needed help. I think the first thing your son has to do is admit he has a problem and hit bottom. And I am sure for my parents, it was hard watching me hit bottom, but it had to happen so I could grow.
Once this happened and the anti-depressants were working, they took me off the anti-psychotics and introduced my ADHD medicines. I also worked with an ADHD coach/therapist who works with young adults and adults with ADHD...and mine is type 2 with impulsivity. From there, it was just learning to recognize my destructive behaviors, and re-learning how to live my life with ADHD. After 4 years I stopped taking my anti-depressants and ADHD Meds and feel a lot better. I still have days where I feel angry or depressed or impulsive and my mind runs a 100 miles a minute, but I know how to recognize those things and how to be constructive. I also have finished my degree and work in Human Resources.
I hope this helps. I think you just need to be supportive of your son. He is going to lash out, but just be there for him. When he is ready for help, just be there for him.
I hope this helps!