I am a Kindergarten teacher so I have seen a couple neat one's. Have him start with an inquiry question about something he is interested in (ie..ask him if he has any questions about how things work)
Some questions I have seen in the past:
Do smarties (the little colored candy in rolls) really make people smart?
together, come up with questions and have people answer them and then try answering them again after eating a smartie.
The one I did with my class last year was:
Why do eggs not break when the mama chicken sits on them?
(It's because of the shape of the egg)
Eggs are actually super strong when pressure is directly on top of them. We ran the experiment by laying 8 eggs on a tray. We layed 1 text book at a time to see how many it would support before the eggs broke. We weighed the stack of books it supported and compared to the weight of an average chicken. This proved that the egg can support the weight of the chicken without breaking. ( We were hatching eggs in our classroom which is why they were so interested in it. )
Good Luck!!