Understanding Exposure seems to be the recommended reading for all us newbie adults. I haven't read it myself but it's probably worth a punt.
I would recommend this.. it's pretty straightforward.. but only if your son is interested in learning the technical side of exposure. You may need to go through it with him, and I'd recommend taking it in small bites.
However.. I'm not sure I would direct a 10 year old towards learning exposure as a concept. I'd much rather direct them towards either a basic appreciation of composition or expose them to as many different styles of photography as possible via photobooks to let them take their own inspiration. This is where you might struggle to find books at the right level.. there's a lot of "Janet and John discover the Rule of Thirds" aimed at adults, and then a big gap to "The Photographer's Eye" by Micheal Freeman, and then a small gap to degree level texts on art/photography.
How about looking for a photobook on the work of
Jacques Henri Lartigue? Lartigue was seven when he started photography and experimented widely. A lot of the themes he discovered (speed, jump) have a very strong appeal today, his shots tend to be fun and spontaneous.. exactly the short of photography that might appeal to a ten year-old. By the way, if you do find a good book on him.. let me know, I have nephews about the same age!
For an older child I would recommend the DVD "The Genius of Photography" produced by the BBC, but it does contain
some adult themes and you should view it yourself before deciding if it's suitable.
