We actually live just outside of Mammoth!  My daughter has never had a problem because of the altitude but her body was born here and is used to it.  I agree that altitude sickness can come on suddenly.  Next time you might want to stop over in Bishop for a night to let your son acclimitize.   I had a fried visit who had to leave Mammoth and get a hotel in Bishop for the night.  The next time she visited she was fine.  Plenty of fluids is key.  Julia