"What issues will the 'oppositional defiance' component produce in our son as he enters his 20s?"
by Michele Novotni, Ph.D.
I'm not sure that this is the answer that you want, but you are probably in for more of the same.
It is often helpful for many parents to continue providing some degree of reasonable emotional and problem-solving support to their grown children with ADHD, especially when additional psychological issues are present. The encouraging part is that you made it this far and have probably developed some strategies that have worked.
I would however encourage you to not do more than you feel comfortable with. As people with ADHD get older, they can also be encouraged to obtain needed support through coaches, psychologists, psychiatrists, counselors, etc. and become more independent and self reliant as they learn to take responsibility for managing their ADHD.