I can recommend Astrid Lindgren. DS (7) adores 'Ronia the Robber's Daughter', but the Karlsson books or Emil books (or indeed Pippi Longstocking) may be easier entry points. His only disappointment with these books is that his friends don't know them - Lindgren is terribly under-appreciated in this country IMO.
Is it too soon for the Little House books (Little House in the Prairie, etc)?
Within Y2 (so age 6) DS very much moved from formulaic series (he enjoyed Beast Quest, Flying Fergus, Time Hunters, Astrosaurs, The Secret Rescuers, Secret Agent Jack Stalwart, Frankie's Magic Football, more recently Bear Grylls Adventures... and would read about one per day in 30-60 minutes) to 'proper' books.
For the latter, he adored the Enchanted Wood series* and much prefers Famous Five over Secret Seven. He's enjoyed books such as Gobbolino the Witches' Cat, Olga da Polga, Fortunately the Milk... (currently probably the most re-read book in our house), and several Roald Dahl, esp Charlie and the Chocolate Factory, Fantastic Mr Fox, Danny the Champion of the World, and The BFG. He loved the first Harry Potter, found the second one scary, loved the third (grown-ups tend to find the third (dementors) scarier than kids do) and read the fourth before I realised - I wouldn't have let him yet! Post HP (or mid-HP rather) it's been hard to find books that engage him, he did a lot of HP re-reading, and reverted back to his favourite formulaic series. Swallows and Amazons (after reading about half TO him first) and Ronia the Robber's Daughter got him going again. Recently though he's got hooked on Percy Jackson, which should keep him reading until Christmas or so ;)
*There's a character in these books that provides an excellent World Book Day easy-to-make-yourself costume: The Saucepan man. I simply pinned all toy pots, pans, kettles, ladles, lids etc that we own to DS' everyday clothing, gave him a colander for a hat, and he won a prize for best costume ;)