Off the top of my head, so far as books:
The already mentioned Narnia series. It’s fairly heavy into the religious overtones, but I doubt a younger fellah would recognize that. I didn’t understand any of the biblical references until my late teens. 
Lord of the rings is a must. I had read the series (from the Hobbit to the Two Towers… I think that’s the last one) by the time I was 12. Loved them, and still read them.
If he’s into Fantacy stuff, i’d also suggest reading the first series by David Eddings (The Belgerad, or something along those lines). The first book is called “The Pawn of Prophecy”.
Isaac Asimov! What else can I say, this man was truly the most amazing Sci-Fi writer to put pen to paper. Yes yes, Arthur C Clarke is good to, as is Heinlein, but I digress. I suggest his Norby the Mixed up Robot series for your brothers age, but with the amount of reading it seems like he does, he could probably go for the Foundation series as well. I wouldn’t give him the Robot series quite yet, as it raises some sexual questions that he might not quite be ready to deal with. I had a hell of a time trying to write a book report on one of those books in grade six. I mean, how do you write a chapter summary for a book that is all about people having sex with machines. In grade 6, that is.
I could go on forever, but I’ll just quickly spit out a couple more authors and titles.
Arthur Randsom - Wonderful stories about a group of kids who have sailing adventures in the lake district in the UK. “Swallows and Amazons” is the first book I read by him, and I still read it to this day seven years later. Other titles are “Pigeon Post”, “Swallowdale”, “Peter Duck”, and others. I swear every time I read these books I feel like I’m visiting old friends. How soppy.
I said quickly… here goes.
Byran Jaques - Mossflower series. Titles are “mossflower”, “redwall”, “Merel of Redwall” and many more. Again, books I still read almost a decade since I first had my grade 4 teacher read one out loud.
Lloyd Alexander… He did the “Black Cauldron” series. Good fun.
Diane Wayne Jones – An amazing writer. Her book “The Homeward Bounders” is a must-read for anyone, regardless of age. I’ve read it literally a couple dozen times. She’s written some other great titles as well (Eight days of Luke comes to mind).
All of the posts so far are also great suggestions. Hitchhikers Guide to the Galaxy is great fun. The “Dark is Rising” series also kicks ass.
There are more I could list, but with only a few posts under my belt I don’t want to annoy to many people. 
As an aside, my biggest wish at your brothers age regarding reading material (I was, and still am a major reader - of both adult and childerens literature) was a gift certificate of at least $100. I don’t know your finantial situation, but I’m sure something like that would blow him away.
Happy reading! And wish your brother a very happy birthday for me.
-niggle
ps. Funny, I just went and looked at my bookshelf. Almost all the books I just listed are on it. Hrm.