I'm so sorry to hear this. Unfortunately, when ADHD remains untreated for years, the symptoms can sometimes grow worse and often new symptoms appear.
Partially this can be due to the stress of living with untreated ADHD "fallout" -- being late all the time, being chewed out, fired, rejected, etc. But part of it is due to bad habits, too -- drinking too much coffee to stay away or ingesting marijuana to relax. Insomnia and other sleep disorders can contribute, too. As can co-existing conditions such as conduct disorder and bipolar disorder.
I wonder why he has never received treatment, despite being diagnosed many years ago. Was an effort made to educate him on what ADHD is -- and isn't? Are you, his mother, now aware of the same?
It is important, if you want to try to reach someone with "in denial" ADHD to truly understand their experience. If all they hear is criticism and admonishments to buckle down, etc. that can drive a wedge in the relationship and, more importantly, make you seem as just one more person who doesn't "get it."
It might help, too, if you learn about the physiological basis for "denial." It might truly be, as you suggest, that he doesn't see that he, as opposed to the rest of the world, is the problem. More specifically, his untreated psychiatric condition is the problem.
I'm not boasting when I say that my book (Is It You, Me, or Adult A.D.D.?) was the first consumer book on ADHD to explain in-depth the reasons for "denial" and how to help someone out of it. I am grateful to the international expert in this area, Xavier Amador, Ph.D, for helping me to write those chapters.
Best of luck,
Gina