I asked my son if it is the act of handwriting that is difficult for him, or thinking of what to write. He said it is thinking what to write. This is something that I had never considered, he talks nonstop and he has ideas galore. Hmmm.
This is exactly where my DD was for most of elementary. We would read out the homework question and she would answer it. Then we would tell her to write that down and she would have no idea what to write. We would take her verbal answer and dictate it back to her - and then she could write it down just fine.
Now, at 11, she's able to write down her own answers without taking dictation. But it's been a long road. We weren't able to access complete testing - we'd have to pay out of pocket, and have no local 2E specialists. If we had it to do again we might travel to get testing we could trust.
For my DD, we have at least two reasons for the problem. 1) she thinks of too many possible answers, and narrowing them down becomes an impossible task; and 2) she thinks in video clips, not words. Translating the clip into words is hard work. Translating the words into squiggles on the page is hard work. Doing both at once was impossible. (These are her own descriptions of the problem, though not in her own words.)