I might be able to help here... (I work at an amusement park here in KC MO)
If his cast is from the elbow down, or knee down, he should be fine on almost every ride. The concern that ride attendants have with casts is the ability to brace yourself, as long as he can hold on, and "push" with atleast 3 extremities (2 legs 1 arm, 1 leg, 2 arms etc) he will be OK and shouldn't encounter any problems.
If it is a full arm/leg cast, same thing remains as long as 3 extremities he will be allowed to ride everything. The only ride I can forsee being a problem is Rockin' Roller Coaster w/the horse collars.
Everything I mentioned above is based on rides I have operated in the past and/or operate currently (flume ride/big red roller coaster) and as many rides at disney are manufactured by the same companies the safety requirements should be the same.
You might also want to take a plastic bag (trash bag etc) with you for the water rides.
Hope this helps.
Jennifer