First of all, you are not alone. These issues are all too familiar.
Since some of his teachers apparently "get it," maybe one of them can try to explain to the others about disabilities and 2e? In my experience, some teachers (including those with sped and gifted backgrounds) don't seem to understand that most of the time a disabled child that is struggling can't do the work for various reasons. Instead, they see it as laziness or stubbornness, and think that the kids won't to the work. They need to clearly understand the difference between "can't" and "won't".
Another option is to have a late assignment provision written into your 504. We are almost ready to ask for this, but so far this year the teachers seem reasonable about late assignments due to struggling with disabilities.
Parenting though is an endless task as you have spend what seems like half your life explaining these things to people, and trying to be patient in the mean time. But its worth it of course. Your DS seems like a good kid that just needs some help and time.
