Try fiction books like Percy Jackson series or Harry Potter. My DDs will read fiction books anytime compared with nonfiction books. My DD7 finished those before her 7th Birthday and she would read 3-4 hours a day. All I had to do was buy the books. I had to ask them not to read during meals and while using the bathroom. I have to bribe them if I want them to read nonfiction books.
If your school has AR (accelerated reading) program, they can earn points by answering the questions (sort of like comprehension test). They have AR points for almost all fiction books. We reward them for their AR points.
The only downside is that they have not found any books they enjoy as much after HP.
Sir cumference series is good for Mathy kids (probably your DS already read those). Life of Fred series will be good but those are for Math purposes and not reading.
