Our fifth-grade son goes to a gifted class one day a week and is also in the Math Club. He does best in math because it doesn't require any writing. If he has a research paper or project to do, it is a major fiasco to get him to write anything down. I've typed his papers as he stood beside me, dictating the information, and the reports were well-spoken and arranged. However, when he has to do any type of writing he will sit for 45 minutes and not have one thing written down. He doesn't mind doing the research or reading but the actual writing is the problem. Is this common? I'm especially concerned since middle school is coming up, and he'll have more than one teacher and many projects.
It is not that uncommon for very bright kids to dislike writing. For some, their minds work much faster than their hands can move and ideas are hard to capture. For others, problems can be more acute: motor planning problems, sensory integration issues, or developmental delays in fine-motor systems. An occupational therapist could evaluate your son to see if any of these are issues. I also recommend that he use a computer whenever possible to reduce his fatigue, and improve his speed and quality while writing.