He will probably not be prevented from riding anything by the CMs, but that doesn't necessarily mean that riding would be a good idea for some attractions. There is no list of things that will be off-limits with a cast. Depending on his particular cast, the CM at the attraction may tell him he can't ride some of the wilder things - especially things that have a great deal of movement, like Star Tours, Dinosaur or Primeval Whirl. I have not heard of anyone with an arrm cast being told they can't ride. But, it is always a possibility and would be for his/other guests' safety if he is told not to ride.
The more wild rides have a little red triangle icon marking them on the park maps. If you want more information about the individual rides and why they have warnings, there is a link to the Guidebook for Guests with Disabilities for each park in the disABILITIES FAQs thread near the top of this board. Pretty much, WDW puts out the warning and it's up to you to decide if the warning applies to you. The red triangle icon is also on a sign outside of the line for each attraction with a small description of the ride.
He should check with his doctor about which rides he should avoid to avoid further injury. Some of the more wild rides have a lot of sudden changes in direction, which may bump his arm around a lot (look for the ones that are listed as "turbulent")