I'm not sure if this will help at all, but FWIW...Writing relies heavily on working memory. This was a major issue with my son, unable to get even a couple of words on the page without crying. Certainly wasn't able to write down novel thoughts; mostly copy work, and even that was torture.
He was recently diagnosed with ADHD and started on medication. Within a week, he was writing almost as well as the other students in class, only falling behind in spelling. After a month, he is currently writing a novel at home on his own, writing short stories at school, and making A's on all spelling tests. He's still got a little catching up to do with neatness and spelling unfamiliar words, but it's like a switch was turned on. People with ADHD usually have poor working memory and visual processing skills which makes it difficult to hold ideas and info in their minds while simultaneously trying to get them on paper.
I'm not saying at all that this is the issue with your child. I'm just making the point that writing and working memory are connected, so anything you can do to improve working memory should also help writing. There are exercises to stimulate working memory and executive function, which, in turn, would likely improve writing and spelling skills.
