You are absolutely correct that your son will be able to ride the majority of things at WDW, and very few at Universal.
Rides I would anticipate issues with:
Haunted Mansion - bar won't close with any significant girth around "the middle"
Space Mountain - if he's agile, he might be OK to climb in and out, but the ride vehicles are LOW to the ground, and can be hard to get out of once in. I just don't ride as it's not a good enough ride to deal with the hassle.
Teacups - the spinning plate will hit right at mid-abdomen level, and be very uncomfortable if not impossible to spin.
It's a Small World / Pirates - Not a ton of leg room in these, but they are manageable.
Everything else is pretty much fine. I am shorter than your DS, but close in weight (I weigh less) with almost all of my weight in my belly and rear. I can make pretty much any other ride at WDW work with a little maneuvering accept for the ones above. I would even go so far as to say he might be fine on Space Mountain if he is able to get up and down easily, but I have a bum knee and much prefer Space Mountain at
Disneyland anyway so it's just not worth it for me.
Universal is a completely different story. I ride about three things there as motion and movie rides (like Star Tours) make me horribly nauseous, and none of the coasters are designed for anyone of size. I CAN ride Spiderman, Men in Black, Despicable Me, the Simpsons ride and most of the stuff in Suessland, but I pretty much only ride ET and the Cat in the Hat because everything else makes me sick.
I live about 2 miles from Universal and have APs, and I'm a CM @ WDW so let me know if you have any other questions. I'd love to put your fears to rest so that you can focus on having a magical time!