We have had a similar problem finding things for our ds. Although he is older than your ds, here are a few selections which I think stretch down to the range you are trying to hit - Good luck!
Great Rhyming one for Halloween or anytime: - Frankenstein makes a sandwich - you and your ds will be ROFL!! (We were, and I don't think it's just because we're odd
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Also, for more poetry fun: Prelutsky's 'Pizza the size of the Sun' and 'It's raining pigs and noodles'. They are silly, silly, silly, and text and pictures are very cool too.
Calvin and Hobbes - you might find your ds sleeping with these - we have bought a few compilations and they are at the library as well. I think they have helped in ds quest to develop his sense of humor.
Riddle books, though I don't have a specific one I can recommend off the top of my head.
I second wholeheartedly Captain Underpants - my ds enjoyed hearing these over and over from ages 4-6 and then re-read them himself many times from 7-8. They are pretty thoughtful, hilarious and very supportive of that 'more imaginitive' child...fun illustrations which break up the text into bite-size chunks.
Throw into the mix Franny K. Stein, Little Girl Mad Scientist - DS loved these as well, nearly as much as C. Underpants.
DS also loved Shel Silverstein; The Missing Piece is a big fav; he still cracks up when he gets to the part which was 'too pointy'. Great simple, funny drawings.
Just curious if he goes for music, and if so, what sorts?
