It might be a little below him at this point, but the Wayside School books got my son really into reading, though that was at 5/6 years old. I read those books about a thousand times when I was a kid. My son is 7 now and is really into the Diary of a Wimpy Kid books, which are really quick reads. Kids his age like the Geronimo Stilton books quite a bit, it seems. Mine likes the House of Robots books, Encyclopedia Brown, Goosebumps, and really liked the first Harry Potter book. And yeah, the "I am. . " books are great. He's currently reading the Mrs. Piggle Wiggle books, but those are kind of lame. ETA: the magic tree house books are fine, too. Get a little bit of historical fiction and it should be an easy read if he's an above average 7 year old.
It's also Book Fair week at school, which is kinda fun. He really wants the Guinness Book of World Records and a book about the presidents. NEEEEERRRRRRDDDDDD.