Sci Fi for 11 yr old...?

DD really enjoyed the Dragonsong trilogy by Anne McCaffrey - more fantasy than sci-fi - but very good.

I loved the Tripods series when I was a young teenager, but that was a very long time ago!

If he hasn't read Diana Wynne Jones he might like things like the Homeward Bounders and The Power of Three (again probably more fantasy than sci-fi, but not wizards & magic type fantasy).

"A Wrinkle in Time" by Madeleine L'Engle is a classic.

Terry Pratchett (start with Tiffany Aching)

I'd have said start with The Bromeliad Trilogy, but I would, wouldn't I?
They are IMO the 'youngest' TPs - might even be a bit young for the OPs DS. Also the Johnny books (Only you can save mankind, johnny and the dead, johnny and the bomb.) And the first Discworld I'd go for would be The Amazing Maurice and his Educated Rodents.

I used to love Asimov as a teen, don't recall them being too complex and I wasn't really 'into' SF, it was just because my DF had them on the bookshelf. DS has really enjoyed the Hunger Games, even though he wasn't sure when he first saw them (maybe because of the female protagonist?!) so would def recommend them.

I think the ones I'd suggest have already been mentioned - David Eddings, Terry Pratchett (start with Tiffany Aching), Asimov for real sf - short stories like I Robot, and The Rest of the Robots will give him a feel for them. John Wyndham is always reasonably "safe" for younger readers, he doesn't do sex and gore.

Robert Swindells.
Brother in the Land is about a nuclear war breaking out.
Eater of Worlds is good for that age too - he's written quite a few sci-fi ones, some are for younger readers and some are aimed more at older ones. Daz 4 Zoe is set in a dystopian future but might be a little bit old for an 11 yr old - the couple fall in love (I can't remember if they actually have sex or not, it's not explicit, they snog a lot but it might be a bit more than you would want him to read yet)
I second John Christopher - Tripods Trilogy and Prequel, the Prince in Waiting trilogy, Wild Jack, just about any of his except The Pendulum Swings, that isn't really a kids book at all I don't think, and Wrinkle in the Skin deals a bit with rape of some female and a male survivor with sn, so that could be a bit much for an 11 yr old. (Again, it's not explicit at all, not even a "rude word" but it is upsetting)
Louise Lawrence, Children of the Dust, Patchwork People, and Calling B for Butterfly.
Monica Hughes has written LOADS of great books - I love Invitation to the Game, but every one of hers I ever read was good.
HM Hoover if you can find them - The Children of Morrow and its sequel, and This Time of Darkness are the best, but again, all hers are good. The Lion's Cub isn't sci-fi though, it's a historical one based on real people. All her others are sci-fi.

Asimov will be fine. Also Arthur C Clarke.
Also the Heinlein juveniles (but watch out - any book thicker than about an inch is his dirty old man period).
Orson Scott Card - Enders Game is absolutely fantastic.
If he likes fantasy try the Belgariad by David Eddings.
The Amber books by Roger Zelazny.
Edgar Rice Burroughs Mars books (I read all of these around that age) - get him to pick holes in the science for these!
Trudi Canavan The Apprentice etc.
Definately John Wyndham - we did the Chrysalids at school year 8
For slightly older/braver - Michael Grant Gone etc

My DD read all the Tiffany Aching books by Terry Pratchet and loved them. I think that the other Discworld novels may be a bit too gown up for her. She's now started on Artemis Fowl and has Percy Jackon lined up for after that. If these don't appeal how about Wyndham? I loved the Midwich Cuckoos.

Asimov is good, but make sure the print isn't tiny. I'd probably try short stories first.
I would avoid CS Lewis (much as I love his other books) as his SF annoyed me, because the lack of science.

Check out John Christopher: wrote a lot of kidlit sf. John Wyndham wouldn't be too old for him either, I'd reckon. Jean Ure's Plague 99 and sequels are good too.

Hitchhikers guide.

Has he read the Hunger Games? Very popular in this household (I have an 11 and a 13 year old). There are quite a few books in the same sort of genre.