I wish I had a suggestion for you. We had the same problem with my DS7 last year (when he was in public school--we are now homeschooling). The county rule was that he couldn't advance in reading until he matched his reading level in writing skill (not actual handwriting, but being willing to write answers about what he was reading). This was very frustrating to him, because he was being forced to read Judy Moody at school when he was reading medical books (written for med students) at home.
He once wrote on a paper that the questions did not deserve to be answered because they were so obvious. We (teacher & parents) tried to explain that he just needed to answer them to prove he understood the literature. He did not understand why you would have to prove that you know the answer to such simple questions--he just thought they were way too obvious.
Now that he is homeschooled, I've started a new writing program with him that has a very scientific approach. It has taken his fear of writing away, because it is VERY structured. I think he likes it because it helps give his thoughts structure. He definately thinks faster than he writes.
Good luck working through your son's issues. My heart goes out to you!
