Do you have a good used bookstore nearby? When my ds was that age I used to take him with me and we'd just browse the science sections together and I'd let him pick out books that he was interested in. That worked a lot better for us than me trying to guess/guage what level would work best. He also really liked watching Nature/Nova etc
Another thought is sneaking a peak at a middle school science book if you can find one. Our district's middle school science books are about at the level ds would have enjoyed reading for fun when he was in 3rd/4th grade.
Sorry I don't have any specific titles for you -
polarbear
