My child also has ADHD, which impacts his writing and organization, and is in 4th grade. He has an IEP. We're still looking for the perfect solution, but having checklists helps a lot. He has one editing checklist -- i.e. sentences start with a capital letter and end with punctuation, other grammar and sentence level issues -- and then his teachers are supposed to create a checklist for each assignment, such as paragraph has X number of sentences, include Y details about topic. Also, he brainstorms with the special ed teacher, using a graphic organizer and the special ed teacher as a scribe, so he can get the thoughts down more easily.
But it's a struggle.