Generally speaking, you can't go far wrong on sci-fi for a tween if it was written before the 70s, as the violence and sexuality were a lot more muted back then.
I was a major sci-fi and fantasy buff in my youth, and when DS was really getting into the sci-fi, thought I had it made with shelves full of ancient Heinlein, Asimov, et al. As Dude notes, little explicit adult content in the old stuff. But.... a caveat for younger kids. The sexism can be brutal - all the more so for being casual and unintentional reflection of the times, not deliberately misogynistic - and the racism iffy at times as well. When I re-read a stack of Asimov short stories, for example, I realized there wasn't a single female character who wasn't utterly ditzy, and usually selfish and greedy to boot, with the except of the occasional serious neurotic (Asimov didn't date much
). Heinlein is better for having some strong female characters - but it's also constantly emphasized how utterly unlike normal female-kind they are.
Now, I devoured this stuff at half my DS's age - but I was a pretty sophisticated absorber of info. He, on the other hand, is astoundingly naive, with a tendency (rapidly diminishing, finally) to be a little too credulous. (He's also utterly incapable of discussing his feelings, so reading and talking over difficult topics doesn't work too well around here). I just couldn't give him most of this books until I felt like he had a little maturity and discrimination under his belt, to be able to understand the books in the context of their time, without unquestionably absorbing their attitudes. He's now 11, and I am slowly starting to open up the pipeline. So caveat emptor! While these books were great for me as a young kid, I did feel DS need to grow up a little more before I shared them.
P.S. It's actually interesting to look at how the female characters in Asimov change over the 50 years or so he wrote. Reading the whole pile of Foundation all at once, with a parental lens on, was a fun study in cultural change.
