First of all you should get the doctor to give you a thumbs up on playing. If the doctor says it is okay to play, than you will need to pad the cast. Cal South's rules just say the equipment worn must be safe. Ask the doctor for a sports/waterproof cast. Insurance may not pay for the extra $100-$150 cost, but it is well worth it. The sports cast is lighter weight and can get wet. My DD swam in the ocean with her sports cast and took showers daily with it on. Now, go to Home Depot and get a couple 6 foot sections of black hot water pipe insulating foam. Cut the foam so it extends at least 2 inches past each end of the cast. The foam has self adhesive tape so it will stick to itself. Wrap the cast with black electrical tape. This foam is dense closed cell foam and will provide great protection for the arm and anyone he may hit with it. It will actually be much softer than an unpadded arm and safer to other players. Now pray that you get a referee with common sense. If you the referee will not let the kid play, than have the coach or manager, with kid, go discuss it with the field marshal and referee coordinator. I saw this last one work very well last year at National Cup when the referee refuse to let the player play. The referee coordinator and tournament coordinator said the kid could play.
My DD has used the sports cast and the black foam twice. She was always allowed to play.