DS is 7 1/2 so having a think...
Currently he's reading Percy Jackson books. Devoured the first series (5 books) but got a little stuck on the second series, I think it's a bit 'older' so less appealing. The good thing about Percy Jackson is, it got him hooked on Greek mythology, which leads to lots of further options. D'Aulaires' Book of Greek Myths was the stepping stone to 'real' mythology. Currently reading a child-friendly version of the Odyssey... And once you're into mythology, you can expand to Roman/Egyptian/Norse etc mythologies, and you start realising how much those ancient stories still infuse our everyday lives, language and culture... (you can probably tell I'm a fan!)
Before that we had some Astrid Lindgren, particularly Ronia the Robber's Daughter. And before that, it was Harry Potter. I meant to limit him to the first two, but before I knew it he'd read book three and four too. Now he's impatiently waiting to turn 8 as that's when I said he'd be allowed to read book 5. I'd say the HP books are no challenge per se, the language/writing is pretty simple. From book 4 they become rather long, though. Book 1 is perfectly suitable for a 7yo I'd say, book 2 is seen by many as the scariest, book 3 seems scary for grown-ups but often not so much to children, book 4 is where you get the first deaths (of 'goodies' rather than 'baddies'), and also where themes such as romance begin to crop up.
DS especially enjoyed HP because all his friends where reading it, and all their games were HP inspired. But if that is not the case for you, there is nothing wrong with waiting a bit longer and then perhaps reading them all in one go. Though making DS wait between books 4 and 5 has also meant that he has gone back and re-read, and explored the stories via other media (videogames, audiobooks). Letting him appreciate the books more than if he had just whisked through them all in one go.