Don't get Heinlein's juveniles and his later adult work mixed up -- not only do the latter they have adult themes, they have *very weird* adult themes (reading _Sail Beyond the Sunset_ at 16 was quite the experience -- combining it with _Friday_ at 14 was probably not the best idea, although I was too young, too clueless, and too much of a social outcast in HS to really get into trouble). The juveniles are entirely appropriate for middle schoolers, and _Have Spacesuit, Will Travel_ introduced me to what remains my favorite genre.
Golden Age SF (30s-50s) should be safe on the sex front, have acceptable levels of violence, but while I gobbled those up in middle school these days I cannot take the sexism and the racism. Asimov might be the exception (I so loved Susan Calvin!).
I'd second all of lucounu's recommendations and add:
The Hitch-hiker's Guide to the Galaxy, by Douglas Adams
Agatha H. and the Airship City, by Phil and Kaja Foglio
(if he likes comics the comics version predates the novel, and is available online)
The Harper Hall trilogy (Dragonsong/Dragonsinger/Dragondrums), by Anne McCaffrey
Ender's Game, by Orson Scott Card
(Card's other stuff should be taken carefully, Speakers for the Dead gets more mature, Xenocide is just plain weird, and I found Songmaster and the one Alvin book I read very disturbing in my late teens).
If you asked your son what his favorites/least favorites/weirdest reads have been it might be possible to tailor more recommendations (I skimmed some of the contents of my shelves with an eye to giving some to my son, and realized that what I thought of as "young" had some rather mature themes for a 7yo).
Oh, and does he like fantasy?
