I agree with mrz and don't think it's exceptional just the natural reading process.My twin boys were free readers in rec so who knows what reading age they were. I never gave it a moments thought just concentrated on what they were reading and encouraging enjoyment.
Reading is just reading, the important thing is fostering a love of reading as without that they won't read and they will drop. You need to read and read to keep improving and you'll only read and read if you love it.
Challenging can put kids off and to be honest if they're reading a wide breadth of books you don't need to worry about how challenging they are. It's far more important that he's enjoying it.
Not sure I agree with the sniffy views towards retro books.One of my twin boys(just turned 7) adores Enid Blyton,he's nearly read everything she's written. He's just spent all his bday money on a stash of Famous Fives. Neither of my two got on with Jeremy Strong,they liked a few but not all of them.They adored Flat Stanley too, all of them. They liked /like Roald Dahl,Horrid Henry, Spiderwick,Jack Stalwart,Dinosaur Cove,Daisy,Naughty Little Sister,the Magic Children series,Charlie Small,Wimpy Kid,Yuk etc,etc
Yes they could read far more tricky books but to be frank I think you'll struggle to find age appropriate,challenging and books they love all in one hit so to speak.
Twin2 reads at every given opportunity,he asks to go to bed early so he can read,thats because he loves books.It's priceless and encouraging that does far more good than challenging children when they get to free reader stage imvho. I actually think if they read books too challenging even though they can it doesn't help with fluency and comprehension, they don't get into the rhythm so to speak.
I make sure my boys read something simple every so often to keep things ticking over. I'm also concentrating on the comprehension as I know school are hot on it. You can get very fluent readers who can't tell you what happened in a chapter they've just read.
Actually twin1 is more into facts and Star Wars,he's not a book worm like twin 1 so as long as he enjoys what he's reading and is reading I'm not too bothered.