When I was 11 I read through the first five Amber books, great high fantasy series by Roger Zelazny. First book is called Nine Princes in Amber. Also, Wrinkle in Time should be read by every kid, maybe look into Piers Anthony's "Xanth" series. Some of that stuff might be a bit difficult reading for an 11 year old, but I don't know. I got my 11 year old cousin the first Hitchhiker's book because I remember loving that book in fifth grade, but the family looked at me as though I was insane for getting that book for him. I remember my brother loved Anne McCaffrey's "Pern" series. I generally figure sci-fi fans will also have a taste for fantasy, and fantasy tends to be a bit less of a slog because they don't feel the need to try and justify all the bullshit they're doing, they just do it.
I read some Star Wars EU stuff (specifically the Jedi Academy trilogy), but in a lot of ways you'd do your brother more good to give him something new, rather than just reinforce stuff he already knows he likes. He'll find that stuff on his own. Your job is to show him stuff he generally wouldn't see. The Amber books are really good, they can be a little tough, but they've got some great alternative world stuff going through them and are, in my opinion at least, good for both adults and kids.
