Monkeyman and O'Brien had some great stories, but they're not published anymore.
A lot of tie in comics to children's cartoons. Like Adventure Time or Last Airbender comics.
Adventure Time is good, as are the
Bravest Warriors and
Marceline and the Scream Queens, all by the same creative crew.
TS, how old is your son?
Among series, mini-series and TPBs currently being published, I would suggest:
The Stuff of Legend - A boy gets kidnapped by the Boogeyman who lives in his closet and his toys band together to go rescue him. A little dark and there is some violence, but this is about as good as American comic book storytelling gets, period.
Atomic Robo - Stories of Atomic Robo and his team of Action Scientists. Good, clean fun. Some violence but it's more action than violence.
Bandette - Just started reading this series. Art and storytelling aesthetic that's reminiscent of Belgian/Euro comics from the 50s and 60s. Safe for kids.
Courtney Crumrin - The story and themes in this series is fairly dark but otherwise, good story.
Cow Boy
Ozma of Oz - All ages comics.
Mice Templar is a good title, too, if you're into anthropomorphic mice heroes ala NIMH and Redwall.