I've been surprised how violent or scary some of the G rated movies can be - especially Disney ones with a villain. Here are some my family have found to be pretty tame:

This site may be helpful: http://www.commonsensemedia.org/movie-lists/ages-2-4

I'd be sure to screen any movie before letting your son watch it, as you know him best. These movies work for our son, but all kids are different. I was surprised to find some of the movies my newly 4 year old son enjoys to be rated age 5 and up on the common sense media site and some of the ones I think are too scary are rated 4 and up.

My Neighbor Totoro (sensitive issue - mother is in a hospital, however happy ending)
Ponyo (my new favorite movie, but definitely screen first - some suspense & a supposed villain who turns out to be good)
Sesame Street Dinosaurs!
The Bee Movie (adult themes but not scary)
Wall-E (some suspense, and the scene in which eve tries to shoot wall-e at first)
How Do Dinosaurs Say Goodnight (and other dino stories) Scholastic video collection
Horton Hears a Who (pretty accurate to the book if that is not too scary)
Toy Story 2 is less scary than TS3. TS1 is too scary with the creepy neighbor boy

Here Comes Science DVD/CD by They Might Be Giants is awesome. It's not a movie though - more like a music video of the songs which are all educational and fun. If your son likes dinosaurs, he'll love the "I'm a paleontologist" song.

TV / Videos:
PBS Kids, Nick JR, Little Einsteins