Hum, i work with a nine year old boy who loves insects, dinosaurs, and evolution. Maybe a visit to the Natural History Museum in your town would do the job?

David; I find that sketching people interacting together lots of fun. I enjoy watching people as anthropological case studies and their body language and behaviour in public spaces most insightful. Their bodies and interaction with the physical space around them is worth a try.

How old is your son? Perhaps, if people are too difficult then physical spaces and architectural structures are easier to begin with.