Hmmmm, tough one. Definitely DON'T ask anything that will piss him off
.
1. Ask about his condition and exactly what it entails.
2. Ask about how his condition has affected his life in different stages, i.e. childhood, teen years, adulthood.
3. Does it prevent him from participating in normal activities, i.e. school, socializing, working.
4. How does his conditions affect his emotions?
Good luck with the paper, bipolar disease is definitely one of the more interesting (for what that's worth).