Seconding the advice you already got - WDW has family rides, not the kiddie type that you might be familiar with. There won't be anything that he can ride that you can't. Also, at WDW, you have to be at least 7 to ride by yourself, so they would not let your older child be alone on a ride with him.
Here is a
link to the official Disney website page about touring WDW with disabilities. At the top of the page, there are links to information about the rides/attractions at each park.
Another thing that might be helpful is to get a guidebook from the library or a bookstore. There is a guidebook especially for kids that talks about the rides and has pictures.
Just a few things I can think of that he would probably like at Magic Kingdom. I'm leaving off things that are dark, scary, have a height requirement or would probably not be that interesting to children 6 and under:
Fantasyland:
Small World - a gentle boat ride with large dolls dressed as children from around the world. Some kids with sensory sensitivities don't like this ride though because there is a lot of stimulation with all the dolls and the music.
Carrousel - an old fashioned merry go round
Dumbo - ride cars shaped like Dumbo, the flying elephant, go around in a circle flying thru the air.
Winnie the Pooh - you ride in hunny pot cars thru storybook-like settings
Mad Tea Party - if he likes spinning, this is a great ride for that.
Mickey's PhilharMagic - a 3D movie. Don't know if he's like this or not, but it's not scary and your other children would enjoy it
Mickey's Toontown:
There are a lot of things to see and characters to meet in the "houses" of Mickey & Minnie and Donald's boat.
Tomorrowland:
Indy Speedway - Small, gas powered race cars. You need to be 52 inches tall to drive alone, but a smaller child can ride if mom or dad does the driving.
Tomorrowland Transit Authority - this is an elevated tram that drives around Tomorrowland. A nice, gentle train ride.
Buzz Lightyear - this might be overstimulating with a lot of motion going around you, but the ride car goes slow and doesn't do anything scary.
Main Street: The WDW Railroad goes around the park. You sit on bench seats in train cars. You can use it as transportation to go from one part of the park to another, or take a round trip.
Adventureland:
Magic Carpets - similar to Dumbo, but magic carpets instead of Dumbo.
Jungle Cruise - a gentle boat ride on the "rivers of the world"
Frontierland:
Country Bear Jamboree - audioanimatronic bears sing country
Liberty Square:
Goofy's Country Dancing Jamboree - includes singing, dancing and audience interaction.
Liberty Square Riverboat - cruise on a steam powered paddleboat.
You might want to stop at Guest Services in any of the parks to request a Guest Assistance Card (we call it a GAC) that would allow his stroller to be handled as a wheelchair. Strollers are not normally allowed in lines or buildings (other than shops). Getting a GAC would help since you would have to be carrying or holding him in all the lines.
