I loved The Martian Chronicles and the 2001 series, by Ray Bradbury and Arthur C. Clarke, respectively. I, Robot by Asimov is also a very good choice for young SF fans. If he isn’t ready yet for those books, he will be before you know it. Out of those, I’d especially recommend Bradbury’s books for someone so young.
If he gets into SF, hard or not (and the authors I mentioned, with the exception of perhaps Bradbury, are all fairly hard), he’ll have a lifetime of reading ahead of him.
I’ll second The Great Brain. My third grader has read the Buninicula books and like them very much. He really enjoys the Wayside Schoolhouse books by Louis Sachar as well. He enjoys the Lemony Snicket books, but his is an 8 year old third grader who reads a bit above his grade level, rather than a 6 year old reading up to third grade level.
The Junie B. Jones series.
It’s a classic, but see if you can find the Oz series by L. Frank Baum. The Wizard of Oz was just the first…there are 14 total. Perfect for someone reading on a third grade level (or college level, even 
)
Oh, and of course, Encycolopedia Brown books. He’ll have fun figuring out the mysteries.
I highly reccomend daniel pinkwater, allthough he writes books for a range of ages from illustrated books for young children, to garrison keileresque adult anthologies.
The books I strongly reccomend that I read when I was that age are The Snarkout Boys and the Avocado of Death, Lizard Music, and the Hoboken Chicken Emergency, very original, very interesting, check out all the 5 star ratings and rave review on Amazon.com.
YES! These were my nephew’s absolute favorites at just about that age. They’re very silly takes on history and mythology.
You might have pretty good luck looking up the Newberry Award Winners from the past. I was reading about one year ahead of grade level and remember fondly the following series from elementary school (some mentioned above):
and the following stand-alone novels:
The following excellent children’s books should not be given or read to the child until he has some concept of mortality. The deaths in these books are real and scary and sad:
And if you want my opinion, the Harry Potter series is a little grown-up for a kid who has only been around for six or seven years. No matter how precocious, there are some things he just won’t understand until he’s a bit older. I would say you should go to your local library, or the nearest excellent public school, and ask their librarian for recommendations. Check the books out, read them, and if you think he’ll dig them, snap up a copy. And remember to thank the librarian by voting “yes” on the next funding referendum. 
And now for something completely different.
Precocious, serious and reads at an advanced level. Reminds me a lot of me when I was a larva. Based on that, I recommend Joe Kaufman’s Big Book About How Things Work. I also have fond memories of paging through the World Book Encyclopedia for fun.
To second Abbie Carmichael, I loved the hell out of Superfudge. Can’t remember much more that I read by way of fiction, though.