I'm dealing with a similar issue with my verbally strong, reading fanatic ds12. When faced with a writing assignment, even answering some questions, he has trouble beginning. Last night I talked with him about it and suggested giving himself permission to be less than brilliant. To begin with anything and go on from there. We'll see. He gets lots of practice so I'm hoping eventually he can move past his fear and perfectionism to be able to write his thoughts effectively and to push through blocks.
He was really put out recently at having to reveal PRIVATE thoughts and feelings for his LA class. Like what he loves, and fears, and dislikes, etc. Really fought that hard by insisting his thoughts and feelings were no one's business.
So I told him he can either look at it as an opportunity to get to know himself better and practice self-acceptance or he could practice his bs skills.