Hi Allison,
It is my pleasure to be able to help with your planning!
If you take a look at the 'Attractions' page of the Walt Disney World website
HERE and tick the box for '40" or taller' underneath 'Height' it will list all of the attractions that your youngest son will be tall enough to ride.
If you then also tick the box for 'Pre-Schoolers' underneath 'Recommended Ages' you will notice that the list of attractions reduces quite drastically.
This is because although your son is tall for his age, some of these attractions may not be suitable for him due to his young age.
As I said to you previously, your judgement is key as you know your son better than anyone else.
I would suggest that you try things like the Tomorrowland Transit Authority PeopleMover and Astro Orbiter first and then think about moving on to other more daring rides based on his reaction.
Wishing you a magical trip!