ps - fwiw, ds is reading the Fablehaven (is that the name?) series right now. He's headed into high school next year. He could have been reading it (based on ability) back in kindergarten. His best friend fell in love with the series in 4th or 5th grade and kept urging him to read them and he refused, convinced he wasn't interested. He forgot to take the book he is currently reading to school a few weeks ago and needed something to read during silent reading time. There wasn't much in his classroom to choose from that he hadn't already read, so he picked up a Fablehaven book and read because he *had* to... and that got him past that first page and voila, he realized why his friend back in 4th grade kept telling him to read them. (I haven't read them, so I have no idea if they really are all that exciting lol!).
Fablehaven is truly not that difficult, but it is exciting. It's stimulating, not because of any tricky words, but because of all the puzzles being solved and magical creatures being discovered and interacted with.
I met the author when he visited near us, and he mentioned an exciting new series he's working on. It's about several kingdoms that each have different laws of physics. I thought that sounded interesting. If your DS likes science, that might be a series to look for in the future.
I'm afraid I can't offer any other advice, but good luck!
