My perspective is that of an unschooling mom to gifted kids, so we have not been bound by schools' expectation of written output -- at least not until my kids chose to enter school as teens. But for what it's worth ....
I've always believed that writing requires two things to begin with: something to say, and someone you want to say it to. Really writing is a form of communication, so without those two basics, there's really no point, and I don't blame kids for not wanting to bother with it after reading a summer novel for pleasure. So much of the writing kids are expected to do doesn't have that meaningfulness and authenticity. My kids wrote freely and prolifically once they discovered reasons to write: to share a walk-through of a computer game they loved, to record a fantastic dream they'd had that they didn't want to forget, to get others excited about reading a book they loved, to work through their feelings about family and social relationships, to request help on a tech-support issue on-line.
Assuming the motivation is there, there's still the craft of writing to learn. My dad was a philosophy professor who graded thousands and thousands of college essays during his career and he always said that the number one cause of poor writing was poor thinking. He was firmly of the opinion that clear and cogent thinking is a necessary and nearly-sufficient pre-requisite for good writing. If you can organize thoughts and ideas clearly in your mind, it is a pretty simple matter to put them down on paper. When I was growing up we were encouraged to take part in lively discussions at the dinner table about all sorts of interesting topics, and knew that when we voiced an opinion we would be expected to back it up with solid arguments, and I have to agree with my dad that this first and foremost built our writing skills.
Then of course there's the business of spelling and grammar -- best learned by most people through copious reading -- and the mechanical aspect of writing letters and words. I'm the mom to a dysgraphic son starting his senior year of high school: he uses a computer for anything longer than a short paragraph and has not been hampered by his handwriting issues in high school. I realize that in elementary and middle school teachers need kids to have legible handwriting to help them evaluate what children know and can do, so your ds probably can't vault to using a tablet or laptop for written work right away. But ultimately handwriting isn't nearly the necessary skill it was in the years prior to mobile devices. If you want your ds to work on his handwriting, I would probably do that as a discrete graphomotor task, using a workbook or copywork, rather than insisting he also compose, and spell, and organize and structure his ideas grammatically.
If it makes you feel any better, my two eldest children wrote almost nothing until ages 8.5 and 11 respectively (and the kid who started writing at age 8.5 didn't share any of her writing with anyone until she was about 13), and they both turned out to be incredibly gifted writers. Both got the highest marks in the district on their standardized 11th grade provincial written English examinations and received awards for their writing.
So I guess my vote would be to encourage plentiful reading and plentiful critical thinking and not stress about the writing. If you can encourage interests that generate authentic motivation for writing, that's great, because it will reassure you that the skill is being exercised ... but I don't think all that practice is really necessary at a young age. If you think he absolutely needs to work on handwriting, I would work with him to find relatively painless ways to do that specifically, separate from composition.