Hi, Beanpole, and welcome!
What does he want to do in the future? I know he's young, but maybe having him think about a future career and the importance of doing well in school might help.
Also, I know there are a lot of books out there for people/relatives with folks with ADHD. Maybe you can find one with such advice. My sons have it (grown now) but they didn't give up on school.
What does his teacher say about it?