Perhaps his teacher is indeed giving her class too much homework, simply because she believes that this is her job. I believe that the vast majority of our nation's schoolchildren are given far too much time-consuming homework, and that the bulk of it does not contribute significantly to their learning.
You cannot continue to stay up until midnight every school night doing his homework with him. You also cannot allow him to continue to feel so pressured by schoolwork and to believe that he is stupid because school is overwhelming him. If this pattern continues, your son will become more and more stressed, physically sick, and afraid that he will fail school altogether.
You must find out why your son cannot keep up academically. Is he lacking fundamental knowledge in his subjects? Might he have learning difficulties because he needs to be taught in a different way than his teacher is instructing him? Your son needs a comprehensive evaluation performed by a team of professionals who can identify and diagnose his current problems. Anything less than this would be cheating him out of the help that he deserves. He doesn't deserve the sickness or the low self-esteem that is plaguing him.