For rides, our three year old girl loved:
MK: Goofy's Barnstormer, Dumbo, the carousel, the tea cups, BTMRR, the regular train, Pirates of the Caribbean, Monsters Inc. Laugh Floor, and Philharmagic. We honestly didn't go on a single thing that she didn't want to go on a second time, though.
Epcot: Test Track, Finding Nemo, Turtle Talk, Soarin', and she liked the last part of The Land, where you go through the greenhouses.
DHS: She loved the Little Mermaid stage show, and she doesn't even like the princesses in general or The Little Mermaid in particular. Other than that, she seemed to like every ride we went on about the same amount. I think she was a little lost on the Great Movie Ride, because she hasn't seen a single movie you "ride through", but she still liked the ride in general.
AK: Didn't have much time in this park, but she loved the Safari and she did fine on Dinosaur, asking to ride it again immediately after we got off the ride.
She is pretty difficult to scare, though, so take ALL of that with a grain of salt. We went to
YouTube before our trip and I let her look at the movies of the some of the rides that are on there. That really helped her mentally prepare for what the rides would be like and took a lot of the fear of the unknown out of the whole experience.
So that's the list of rides, but by far she enjoyed things in the park other than rides at least as much, if not more than the rides themselves. She wanted to look at everything, things that adults usually walk past. She loved all the fountains and I ended up using all the pennies I had (brought to make pressed pennies, but she wasn't really interested in those) giving them to her to throw in the fountains and make a wish. She loved the animal benches at AK. She loved all the details, the signs, the posters, the buildings. She would talk about the walkways when they changed (embedded rocks in some of the AK walkways, different surfaces or paving stones as they changed from area to area). She loved the plants and flowers (it was the Flower and Garden Festival at Epcot when we were there) and just all of the random stuff around the parks. One of her very favorite things were some colorful wind ornaments up at the side entrances to both sides of Future World at Epcot. They're up on the lamp posts and she mentioned them every time she saw them and we talked several times about how colorful and pretty they were. I actually learned a lot from her while we were there about how to really look at the things around me and appreciate the things that were unique and interesting.
Oh, yeah! All of the public performances! She loved all of that stuff. There are a bunch of street bands in pretty much every park and she loved watching every one we found. I think you can get a schedule of those sorts of goings on at guest services when you get into the park, but we seemed to trip over lots of fun public performances during our trip. Lots of this kind of stuff in the Epcot World Showcase.
She also LOVED meeting the characters and getting them to sign her book. I know a lot of kids can be frightened of the characters, but I think it helps to talk about it in advance. Ask what your child might say if they met (fill in the blank with a character they like that they are likely to meet) so they can start to imagine the situation now, then they might not freeze up, but instead, they'll feel confident and prepared. Prepare them for the size of the characters, too. I told my daughter a bunch of times that the characters were much taller in person than they looked in movies/on tv. I specifically told her "tall like your daddy" a few times, because her father is really tall and I thought having something concrete to compare them to would help her. She had no character fear at all from the very first meeting to the last, and I hope some of the talks we had helped her to mentally prepare.
Have a great trip! I hope your son has a great time!