Maybe you could tell the teacher that you are trying to help DS at home with his writing assignments and you need to know what the requirements are for each assignment. Ask about her grading rubrics.
It could be something you're not expecting, like he needs to write a certain number of sentences, the paragraphs need a specific structure or he needs to use more descriptive words. I guess what I'm saying is don't assume that she's grading him on content.
You can always have him keyboard (or dictate) an assignment and then make a handwritten copy to turn in. They'll never know...
