Ok, let me re-phrase my question. If the directions on the school assignment said "Write a paragraph answering this question", would it be a well-organized response that made sense? I think you have to try to separate out whether he could write a decent response if he knew that more than one sentence was expected. If he can not, you still don't know whether it's impaired language that's causing the problem or if it's poor EF (or both). If you find a good language test that is accurate with gifted kids, and it comes out fine, then you can probably assume it's a written expression issue caused either by poor instruction or poor EF.
