Hi Laurie,
Ah, the days when little ones need a nap. I remember those well with my own daughter. You are smart to be thinking ahead about places where your son might sleep for a little while while you are at the theme park. I also love your plan to leave the park when he needs his longer nap, which should help to keep him rested and happy.
It sounds to me like motion is something that helps your little one sleep. The Walt Disney World Railroad would be a good place for him to sleep, since you can ride it continually and would not need to get off until he wakes up. Another idea is the Tomorrowland Transit Authority PeopleMover. This ride is long, typically not crowded, and is gentle enough for a little one to sleep. Another place that my daughter slept when she was little was on the monorail. The great thing about this was we could leave her in the stroller if she fell asleep in it. You can just roll the stroller onto and off of the monorail and even change lines if you get bored with the scenery!
If your little one just needs a place to relax without the stimulation of the theme park, be sure to check out the
Baby Care Centers at the Disney theme parks. They provide nice places to change and feed your little one, or just to take a break.
I hope that these thoughts are helpful for you, Laurie! Have a wonderful time planning your vacation, and please do not hesitate to ask if you have any further questions.