ADHD doesn’t go away. It’s a physiological difference in the brain. If he had ADHD as a child, he has it now, and always will.
Is he open to treatment as an adult? It could make a big difference.
It sounds like he has a poor self-image and a lot of anxiety and shame around perceived short-comings. People often lash out when trying to hide their vulnerabilities because it takes the focus off of them.
This article may have many ah-ha’s for you:
ADDitude Community Moderator, Parenting ADHD Trainer & Author, Mom to teen w/ ADHD, LDs, and autism