The most interesting read ever about this topic for me was Guns. Germs and Steel by Jared Diamond - also Collapse, but that one isn't as focused on the differences In the development of American versus European-Asian societies. However, it's tough reading eve for a gifted 7 year old (not least because of how indigenous American were destroyed not primarily by war, but by disease and why they were more or less defenseless against either. Makes for rather disturbing reading) Though if you want to dive into it with him, you may want to read it yourself and then discuss whatever you feel is appropriate.
