I really understand this. My 4-yr-old has been reading since before 3 but still won't write/color/draw at all and is very stressed out by worksheets. We are actually in the process of getting him evaluated for SPD and possibly needing OT to help him with motor skills. It is my understanding that this could be an excitability in relation to giftedness. I'm not saying that is what is going on with your son, but I offer my support and very much understand what you're going through.