I should have added, my goal here is just to pick some books for the summer that will slightly nudge him to try new vocabulary, slightly longer length, new topics etc. Thanks for your help!
My personal take on this is that for children who are already reading, the best way to build upon their skills are to find a new favorite series that is somewhere between their instructional level and their comfort zone and to let them experience something completely new in real life.
When it comes to selecting books for DD5, her own interest seems to outweigh reading level, hands down. I suspect that what your DS needs is to find something that captures his imagination.
