An artist named Andy Runton has a series of simple comics called "Owly". They use the visual language of comics, but don't require literacy. When the characters rarely speak, they do so in pictograms. The stories and jokes are simple enough for young kids to follow without seeming condescending to adults (i.e. it's not Dick & Jane type stuff). My daughter started on them around that age before she could read and liked them. She's almost 7 and is a great reader, but she still enjoys them. I'm sure there are other good comics out there for pre-literate kids if you ask around. I know the comic stores I frequent usually have a lot of kids books available on Free Comics Day. Stop by and take everything you think might be interesting to your boy and see what sticks.
Once your son begins reading, there are a lot more options: Jeff Smith's "Bone" series, Kazu Kibuishi's "Amulet" Series, Disney Comics (esp. the Carl Barks Duck Adventures). Superhero comics are a bit tougher to get into. Kids are drawn to the costumes and the setup, but the plots and language can be a lot more opaque than you'd expect, especially as they've become oriented towards more sophisticated adult readers over the past 30 years.
Good luck, he's gonna have fun!