Your role is to help your child become a self-sufficient student, not to ensure that he has perfect homework. After helping him create good study habits, stop stepping in, even if it means he gets a couple bad grades. To be successful in the long run, the responsibility to do his work well must be his, not yours.
If the homework headaches continue, you may consider getting a tutor. Learn how to get the most out of your child's tutor by reading this article.