But to get him to write a paragraph....grrrrr! It's like pulling teeth. But yet on the WISC and the WJ-III, his verbal scores were his highest. Maybe that doesn't translate over into writing. When he's trying to write a story/paragraph he says "I have too many ideas in my head at one time, and it's too hard to sort them out." And I can see his frustration with that

Sounds exactly like DS12 at age 7. We actually had 'school incidents' where he would just stall and cry when he was supposed to do a 'write a story that starts with the little half sentence we give you.' Then the teacher said he was 'uncooperative.'
Part of it was that his awareness of standards of what 'good writing' sounded like were much higher than what he could achieve. As he's gotten older he has a better understanding of what is age and grade appropriate, but back then anything shy of "Charlie and the Chocolate Factory" with computer printing was heartbreaking.
What helped:
a) learning to type so that he could get those words out fast. I started bribing him to do 'Mavis Becon Typing' at age 7, and he was functional by age 9, amazing by age 10. It opened up a whole world for him.
b) As he got older, his brain did grow in a way that now that he can use a graphic organizer, he needs one less than ever. He does have a nice big Working Memory, so maybe that is why.
c) 3td grade teacher assigned a writing assignment that was so big and so long that he finally got over the 'it has to be perfect' and into the 'punch drunk - put anything down' stage of writing. I love that stage. He had to write a paragraph for each letter of the alphabet for something that had to do with winter in our State.
We finally go laughing around letter X. Something like: X is for Xray when you slip on the ice and break all your bones. I could tell that his 'inner critic' had gone somewhere warm on vacation and he just wanted to get through those last few letters.
BTW - how brilliant is a teacher who lets the last two weeks before the winter holiday break be all day writing workshops for jumpy little pre-holiday kids? It was so nice not to be having behavior complaint phone calls. I think it was because the kids were all individually as engaged as they could be. My favorite teacher!
Hope that helps.
Grinity