There is a wonderful book, Uniquely Gifted, that is a compilation of essays in support of "provide appropriate challenge to the strengths, and some suppport for the weaknesses" school to though. It's towards the end of the page here:
http://www.amazon.com/Hoagies-Twice-Exceptional-Gifted/lm/UFI3WIS4U6F2I'm not so sure that the written expression part is really a disorder, as much as a normal difficulty of giftedness which "goes away" once touch typing comes in. My son really looked like he "couldn't write" particularly to timed prompts, until the touch typing clicked at age 9. (I'd been patiently bribing him to do some extra work on it since age 7, and they teach it at his school.
Another wonderful book to give insight into his understanding of the writing process is
6 + 1 Traits of Writing: The Complete Guide (Grades 3 and Up) by Ruth Culham (Paperback - Jan 2003)
It's so easy, and worth it, to read this book and read parts to him, and see if he can "think" about writing, or "talk" about writing. He may be quite advanced, as long as you don't ask him to "think" while "printing."
Is my son having organizational troubles in his new placement? sure. Is he getting the extra support he needs? sure. Is it better than if we had kept doing what we were doing? I'm not all-knowing, but I have every confidence that he is getting 100 times more out of this year than he would have without the skip/school change. It seems to me that some kids (ours) need to approach school in a very individualised way, meeting their needs in all their variety.
At the least, you can go sit in on the target classroom and ask yourself, could he do this?
Best Wishes, and keep us up to date,
Trinity