He should have a planner with room to write down both long- and short-term assignments.
A volunteer homework buddy can be assigned to your son to make sure homework has been written down in his planner.
Your son should be allowed to have an extra set of books at home so there are no excuses for "forgetting" to bring home the books he needs to complete assignments.
Teachers can send daily/weekly progress notes home for you to monitor his homework/test performance so he never has a chance to get too far behind in his work.
Teachers can develop a reward system for completing class work or homework.
Your son can use a calendar to plot out mini-steps towards completing long-term assignments.
An excellent resource for other ideas about strengthening organizational skills is Sandra Rief's book, How to Reach and Teach ADD/ADHD Children: Practical Techniques, Strategies for Helping Children with Attention Problems and Hyperactivity. If these goals and accommodations are written on your son's 504 Plan or Individual Education Plan, you should be informed on a regular basis about his progress in this important area.