From what I have been told and I also found on a blog...
http://www.thesportsfannetwork.com/forums/thread/13562-does-the-wcac-allow-repeating-a-hs-grade/?sort=Oldest&view=Flat
Students are only eligible to compete in sports for 8 consecutive semesters once they start high school.
But, I have heard that some kids have received a medical waiver.
Can you talk to your athletic director? You have not even played a game yet so maybe the "compete" portion has not started yet. Just guessing here.
Sorry your son is having a hard time with his grades.