We have the same with DS (8/Y4/ASD). It's been a nightmare getting him to do any writing. After 2 years I finally realise that the school don't know how to teach handwriting and seem to rely on kids picking it up.... DS can't copy others' movements so that wasn't going to happen! I took him to Magic Link lessons then used the DVDs. So just this year have sorted the handwriting.
Having said that we've had a trial of a NeoSmart keyboard for creative writing in class and hopefully typing stories will increase output. Handwriting will be used for other subjects.
However sorting this has simply revealed DS' difficulties with the thinking aspects of writing. He can't really attend to non-preferred topics or order a story to be of interest to others. Character ideas are completely alien....
His teacher last year started him write on his own topics when possible and he also gets to use the computer as a reward.
But I'm worrying that while that sorts one problem for the teacher it leaves me with 2 to resolve before secondary! I need to get him off the rewards and be able to handle others' topics (at least a little bit).
We are about to try out exercises from Amanda Hartington's I hate to Write Stories. We're using Rory's Story cubes. Basically just trying to talk about stories and tell tales before writing happens. I also read aloud to the kids and have been talking about how the story shapes work (YouTube Kurt Vonnegut) and fit to the books we read. Also descriptions in the books and how we might be able to use or adapt these. I also found an Emotions Thesaurus on amazon which lists out ways to "show not tell" how characters are feeling for many emotional states.
No idea how/if all this will work out but that's how we plan to tackle it this year. Mainly trying to get some fun back into it and show how to use ideas from his own experiences. It was Y1 when this was raised as an issue by school and going into Y4 I realise that if we don't sort it, it probably will get worse. Good luck