I'm not suggesting this instead of anything which you've thought of (because you're already looking at some of the best titles for your son's age) but, when I was a kid, I loved the "Observer's" series of books. Much of their content was probably largely 'above me' when I first got the books but I found them great works of reference and, as the years went by, I kept reading them to study the subjects in more detail. (I've still got several of them!)
Alas, they're no longer in print, but secondhand books are sometimes just as good as new ones. For £2.27 (including postage) you might like to consider this as a bonus book for your son:
http://www.amazon.co....e=UTF8&condition=used
Chris