My heart goes out to you and your son :-(
While I haven't been in your situation my immediate thought is the teacher can't be failing everyone, does your DS have a friend who is doing well in the class who would be willing to be a study partner?
Also, some honest comments from your son to the teacher...'I enjoy math, Ive done well in the previous classes (algebra, geometry, etc.), I complete my homework, etc. .... yet I am struggling with some of the exams, what do you suggest I do to be more successful in your class?'.
