Hi Donise!
I've traveled to Walt Disney World Resort with friends and family of all shapes and sizes. While some rides are certainly less roomy than others, I don't think your son will find any rides that he simply can't ride due to his height/weight.
You'll find that certain attractions in the Fantasyland section of Magic Kingdom Park cater to younger guests, such as Peter Pan's Flight and The Many Adventures of Winnie the Pooh. The ride vehicles typically seat two guests, but if your son would feel more comfortable riding alone, he can certainly request to do so (That holds true for any attraction). One ride in particular in Magic Kingdom that he may struggle with leg room (I'm 5/8, and it's a tight fit) is Seven Dwarfs Mine Train. The two-seater cars have individual lap bars that your legs need to fit inside of, and my long legs find it a tight squeeze!
On attractions that have individual seating, you'll often find a "test seat" located somewhere near the entrance. It's always a good idea to hop in and check out the test seat if you're concerned about how well you'll fit into the ride vehicle.
Another tip: As a long-legged guest, I've discovered that certain attractions have front rows with more leg room. I'm never shy to ask for the front row when I visit
DINOSAUR (Disney's Animal Kingdom),
Space Mountain (Magic Kingdom Park),
Test Track (Epcot) or
Rock 'n' Roller Coaster featuring Aerosmith (Disney's Hollywood Studios).
I hope this is helpful in your planning. Have a magical day! - Diane W.