Hi there, Mun Kwong!
That is a questions you will certainly want to discuss with your son's medical team before you come to Walt Disney World.
Each ride features its own set of restrictions and recommendations. From the
Disney Parks Web site:
"Some of the attractions at Walt Disney World Resort have restrictions to ensure Guest safety. Typical safety restrictions include height restrictions, health and safety advisories, and accessibility guidelines. These restrictions and advisories are listed on the individual attraction pages throughout the site."
While it is likely that nothing will specifically prohibit him from riding with a cast, you'll definitely want to consider his comfort level - some of those rides can be bumpy, and you don't want to jostle that healing bone. You'll also need to make sure that the casted arm is never on the outside of any ride you choose to ride.
Please check the Disney Parks site for more information regarding medical restrictions, and do ensure that you meet with any necessary medical personnel before you make those decisions. All rides, of course, are experienced at the rider's own discretion.
Have a wonderful trip!
See you later, celebrator,
-Deborah B.