Sorry to hear about your son. I myself was at WDW with a cast just 4 days after breaking my arm. I wasn't in top form because I still had alot of pain, but nothing stops me from going to WDW!!!
The only ride I had trouble on was Tower of Terror. Mostly because I couldn't get that "death grip" on the seat in front of me!!! I don't do Roller coasters, but do remember doing Thunder Mountain and Splash mountain with no problems. I was never stopped or questioned by any cast members about the cast.
Hopefully, your son will feel better....July is still a few weeks away.
Just a thought, but maybe the characters will sign his cast?!?
Don't know if they're allowed to do that, but would put a fun twist on things. Have fun!!