I read The Lion, the Witch, and the Wardrobe when I was about that age. I tried to read the others in the series, but they just weren't that interesting to me. I think the only reason I was interested in that one was because I had seen an animated movie based on the book first!
I agree that it really depends on the child. If the child is interested in reading it, I say let him go for it. There's really nothing to lose, and if you keep him from it, telling him it's "too difficult" for him, you could discourage him from wanting to read anything.