My son, 8, is a huge Andy Griffiths fan. He loves the The 13-Story Treehouse and The Day My Butt Went Psycho series and has read them all multiple times. The silly, slightly rude and occasionally gross, but not over the top (it's all along the same lines as Captain Underpants), humor is right up his third-grade-boy alley.
The book versions of this "Just" series are hard to find, no longer available or never offered in the U.S., but the CDs, for whatever reason, seem readily available. My son loves listening to these shorter stories before he falls asleep and tells me, "Mom, they're so funny."
I've listened to a few and you definitely need to be a young boy to "get" this humor, but it's all pretty harmless and anything that gets my son reading with the passion Andy Griffiths books do, is a winner in my book.