In defense of teachers:
Has it actually not been taught or did he himself not learn the material (meaning he didn't ask questions when he didn't understand, he wasn't paying attention as many students do, etc.
)
I can't see many teachers not giving homework that they never covered, but I am sure there a few that don't.
Hope it gets better! Maybe there are some after school homework help that your soon could attend? Often there are and he could ask questions and learn one-on-one with someone.. often a teacher. Plus that would give you a break.