I would suggest "Holes" by Lois Sachar. I think it's a grade 5/6 level, but it's an awesome book for boys (as well as girls).
I found some great ideas going to a large bookstore and just looking at what's on the best-seller or reccommended shelves for that age group. Have you read Silverwing/Sunwing and Firewing? Those are popular with my grade 6 boys.
Another good one is called "Dust" (I forget who it's by but it's a new release)./ Hmm... Oh, The Golden Compass is a good one... hmmm