Oy, are you now more confused than ever!? I think you know your child...does he like meeting characters? does he like thrill rides? live shows? is he into animals? each thing you mentioned really caters towards a different child... mine could care less about meeting the big headed characters, kinda freaks them out.
Well, my boys are 4 and 5. We did AK last year, and I have to say it was a bit disappointing. The only ride they did was the one just like dumbo in MK. Bug movie too scary, dinosaur too scary, coaster they were too small for, safari a bit of a yawn yawn especially if they've been to a zoo. We didn't do the rapids as MIL didn't want to get wet. Lion King show excellent!!
This year we're doing EPCOT and MK. I just think they will find more to do there. (Especially at Christmastime). Little things like the plate spinners in China are amazing! The imagination ride, 3d movie, mini version of sea world at the living seas, illuminations is great. We'll probably do just a half day at EPCOT and and the rest of the day revisiting MK.
Sea World good too, just realize it is a lot of scheduled shows. So, you have to get to a certain theater before hand to get a good seat...wait for the start and then sit through the show. One of my able to do that, the other not so much...too young.It was, however, quite relaxed and much less hectic than Disney. DH and FIL did the dolphin swim at Discovery Cove, a once in a lifetime experience.
My kids probably wouldn't like Universal (I love it), I think simulated earthquake, shark attacking the boat, etc.. is a bit too real for them. I think my youngest at 4 might not understand this is just pretend and be quite scared. I guess it depends on your child. Hence, the reason we're not doing MGM yet. I've never been to the islands of adventure section, Dr. Seuss land looks cute, but again a lot of coasters in the rest of the park. Good LUCK!