Have you requested that the school test him? Even as a homeschooler they must do this if a parent requests. You may need to register as a homeschooler to have it done depending on what your state law is. Most schools here use the WJ. This might give you a little more information and would be testing that the school will accept. It might also be the beginning of plan to meet his needs both in areas of giftedness and areas of need.
My daughter has similar problems with written expression and somewhat lower spelling scores as well. She is 2E, and has gifted aspergers and dysgraphia. You do mention quite a few areas that are consistant. Have you read the article I linked to on the aspergers thread about educational needs of gifted aspergers students? You might find it interesting reading. Also reading on dysgraphia and similar written expression disabilities might give you some more information. The LD online website mentions many. Regardless of why he has trouble in this area they have some good information.
Oops, crying baby...
