You say a “full arm cast” - does that mean shoulder to hand? Is his arm bent at the elbow? And probably most important does he wear a sling that positions the forearm in front of him? If he can move his arm outward from the shoulder, he can probably be ok on FoP. It might be uncomfortable if the arm must be held tight to his body. He’ll want to try the test seat.
He should be fine on Navi River Journey and Soarin’.
Enjoy your vacation!