I loved to read as a child and was reading well beyond my age, as is my DD. (She's 5 and taught herself to read about a year ago. She's at about a 9-10 year old level now.)
Be careful not to choose books that deal with concepts or emotional issues that are beyond his capacity. Older childrens' books may do that, and he will have the intellectual ability to perceive the problem, but may not have the emotional maturity to distance himself from it. I find that children who are intelligent beyond their years are also very perceptive and sensitive to conflict or difficulties, and such books may give him more stress than he is able to cope with.

Anyways, here's a few titles that occured to me as I was thinking about it:

- "The Cricket in Times Square"
- books by Marguerite Henry (horse stories, mostly.)
- books by Beverly Cleary
- "Farmer Boy" by Laura Ingalls Wilder
- "Redwall" series by Brian Jacques. (might be a little beyond him yet - my brothers loved this series)
- "Magic Treehouse" series - brother and sister have a magic treehouse that allows them to explore through time and space. Light and interesting reading - lot's of information!
- I'll second what Ryn said about Roald Dahl - great stuff! Loved it as a kid.
- you could try the original Winnie the Pooh stories by A.A. Milne. Don't confuse these with the Disney crap that's so popular for toddlers. The original works are very creative, have great vocabulary, display human nature in a clever and humourous way, and are really very funny. Adults catch more than children do of these, but your son might pick up on it too.
- a good children's encyclopedia/dictionary. (my favourite books as a child!)
- you could try some of the "Dear America" or the "Dear Canada" or the "Royal Diaries" series. They are fictional diaries written by children aged 10-13 or so, set in different time periods and events in history. They might have an Australian series too, I really don't know. These books are written by various authors, and I have yet to find one that I was disappointed in. They are easy to read, and FULL of information about these eras/events. Even if they're not local history, they're still fascinating, I think. Down side is most of them are about girls, but he might not care, and there are some about boys too. (A boy on a 1800's cattle drive in the western US, a boy on a whaling ship in the late 1800's, a young boy slave in the US, etc.)

Broaden his intellect by providing him with interesting books in the areas of science, art, sports, mechanics, etc, etc. But don't make reading a chore - let him read at a wide range of age levels. Don't say "you're too old for that book" if it's one he wants to take home from the library. I'm 28 and I still enjoy reading my kids books sometimes. Reading should develop the mind (I don't believe in allowing my kids to read crap) but it should also be fun!

If I think of any more titles, I'll jot them down for you.