I don’t see a problem in sending the teacher an email, copying the department chair, and saying
Dear Ms. So and So
I just wanted you to clarify the policy in your class regarding work missed to illness so that I understand it. My son’s explanation left me with questions. My son …[politely tell your story showing respect for authority, etc]
It’s not accusatory, it’s just a question. It gives her the opportunity to tell you the policy, and having the department chair on the email chain ensures a response.