Another mom here. My son is 23, living in an SLE after second in patient rehab. I know the desperation you feel, wishing your son would "just get it." Worrying that I (as the last person still hanging in there for him/not giving up) couldn't abandon him because that would be his last chance at getting it.
Although my son is very new in his recovery, he is very clear to me that what really got him to take notice was when "I" drew the line. I really think he respected me for finally keeping my boundary. In our arguments while he wasn't sober, he made comments indicating he wanted me to take a firm stand. Regardless if age, children always desire a boundary. That's how they know where their safe place is. Although they may kick and scream and fight it, they do know deep down inside that if they honor that boundary, they are safe. No more fighting.
My son talks about all the pain he was experiencing while high, trying to self soothe. So many people in his life tried to make him see what he was doing, but he just didn't get it. Like you, I felt like I was the last one hanging on for him. And therein lies the power. Show him you love yourself enough to put a stop to your pain - which means removing yourself from your own child's life - only then will he see the example you are setting for yourself. He may gain respect for you and look inside himself differently. As mothers, I think we have the ability to impact our children's mindset by doing what we preach. Take steps to make yourself better. Do whatever it takes to reduce the pain in your own life. Here!s a big one - put yourself first! It doesn't feel natural and you will need to really work it, but its worth it. Your son will view you differently - not as a quitter, but one who is fixing what is in her power to fix. Lead by example. You will cry and second guess yourself, but it is a journey you must make. For your sanity and for any chance that your son may see things differently than before.
Prayers for you and your family.