My DD9 is working with an OT this summer to address writing initiation and output. They recommend the book Late, Lost, and Unprepared: A Parents' Guide to Helping Children with Executive Functioning [Paperback]
Joyce Cooper-Kahn (Ph.D.) (Author), Laurie Dietzel (Ph.D.) (Author)
They break down different types of executive function impediments to writing and have specific recommendation regarding software and other accommodations.