LOL!!
I'd go find some good used non-fiction books by Isaac Asimov.
http://en.wikipedia.org/wiki/Isaac_AsimovScroll down to the non-fiction part. Start with the first one (Fact and Fancy) and go from there. They are a good general introduction to classical physics, scientific method, general science ideas, atomic physics, light, etc. Very readable for HG+ kids. You will enjoy them yourself.
I'd see about buying them used as they should be re-read several times! I know I wore my copies out!!
When he can do Algebra, I'd get him a set of the Feynman Lectures on Physics.
Rent the Cosmos (Carl Sagan) DVDs now.
The problem with string theory is that its still a conjecture, same as dark matter. The latter is looking like its on its last legs.