Library worker and mom to a 3rd grader here
Cressida Cowell's books are very popular right now. Especially with
How to Train Your Dragon out in the movie theaters. DS started with
How to Be a Pirate, poured over
How to Train Your Dragon, and is now reading
How to Speak Dragonese over Spring Break.
DS loved the
Diary of a Wimpy Kid books.
He has read all of the Magic Tree House books - including the non-fiction Research Guides that go along with some of the later titles.
Let's see, other series that are good for guys that age:
A to Z Mysteries by Ron Roy
Choose Your Own Adventures
Short and Bloody History of... (vikings, ghosts, pirates, etc.) by John Farman
Dragonslayers Academy by Kate McMullan
the Great Brain by John Dennis Fitzgerald
Bailey School Kids by Marcia T. Jones and Debbie Dadey (Pirates Don't Wear Pink Sunglasses , Dragons Don't Throw Snowballs...)
Jigsaw Jones Mysteries
Blast to the Past by Stacia Deutsch and Rhody Cohon
Time Warp Trio by Jon Scieszka
Here's a good link for other series:
http://www.mcpl.lib.mo.us/readers/series/juv/title.cfm