DO NOT let them chew you out about his homework. Are you doing the best you can in the situation? Is he doing the best he can? If so, then that's all you can do. There comes a point where the frustration that arises in both mother and child invalidates the whole point of doing homework. When he's that frustrated, he not learning anything. I have 2 kids with ADHD and there have been many times that I have sent homework back to school unfinished with a note saying how long we had worked on it.
I know that getting a 504 plan can be a long, frustrating process. You have to stick with it though and make sure that the accomdations are the best for your child. A 504 plan is valid through college. Somethings to consider: alternative punishments and less/different homework, and being allowed to do some work on the computer. Typical school punishments for your son's age include taking away recess and ISS. No recess just compounds hyperactivity and, at least for my son, ISS isn't a punishment because he likes it there with no distractions. For homework, even just completing half will really lessen the frustration while still reinforcing concepts taught at school. Many kids with ADHD have fine motor issues leading to handwriting difficulty. My DS10 still writes like a 1st grader. Being able to do some (not all because he does still need to practice handwriting) schoolwork and homework on the computer has also helped with the frustration and really cut down on the hours spent doing homework.