Try Sweet Farts by Raymond Bean. It's a silly title but a very smart book. My son and I laughed very hard through this one. The main character tries to cure the smell of farts for his 4th grade science fair project. I don't want to give away any funny moments but a real letter from Benjamin Franklin plays into the story and his grandmother is hysterical. Let me know what you think.
My son likes Captain Underpants. He also likes Where the Sidewalk Ends, a Light in the Attic (Shel Silverstein). He loves Garfield Comic books. He also likes Bone Comic books (Jeff Smith). He also reads Hank Zipper's books and loves anything with a comic flare. Hope this helps.