I always like to recommend Encyclopedia Brown. Of course, they are short stories, but they challenge you to "figure it out" and get you working on critical thinking skills. A wonderful supplement for the homeschooler (which is also me!)
Anything by Beverly Cleary is great, although he may not appreciate stories about Ramona...she has lots of other wonderful other stories.
I also love the classics, and think they have great value even at a young age...just choose based on what he may be interested in. Tom Sawyer, Huckleberry Finn, Around the World in 80 Days, White Fang, Arabian Nights, etc.
I'd have so many more recommendations for you, but they are more tailored towards girls.