If he's in that Magic Tree House series-mood, there's several other series that are just a slight step up and might appeal, while getting him more willing to branch out a bit. Magic School Bus chapter books (not picture), A to Z mysteries (yes, there's 26 of them), and Jigsaw Jones mysteries come to mind. These are probably easily found at most libraries; the first two can usually also be bought in bulk via Scholastic.
Like others, I read to my kids every night, and learn a lot about their reading levels by which books they finish on their own, and which they leave for me to read.
