With so many different teachers, your son may miss the more personal guidance he previously had from an individual teacher. Have him attend a conference with you and each teacher who failed him so you'll get a clearer picture of the situation and he'll understand what he needs to do to improve his grades in these classes.
Teach your son the two big secrets of being an organized student: using a large binder with pockets and carrying a book bag. Organize the binder with him so that there is a special section for each class as well as a pocket for papers he hasn't had time to organize. The front of the binder should have a special zipped pocket for pencils and other classroom supplies.
Set up a time each day for helping him organize all his class journals, folders, and papers in the binder. Work with him until he has learned to keep the binder organized. Be patient -- it may take a while.
Give your son a book bag for all of his books and the binder. Since he is rushed between classes, he should stop at his locker twice during the day: once for his morning class materials and once for the afternoon's. He should keep the binder in the book bag at all times.
Whether your child brings anything home to study or not, he should be sitting down in an area where you can see him and spending 50 minutes each day on school-related work. Ask his teachers to help you select books and other stay-at-home materials so he will always have something to do.