I think he is naturally smart. Slow in writing is only because of his lack of interest on the school subjects -- I believe, you would have observed cases he writes fast when he likes the subject. From the way u explained he is blessed with amazing memory and analytical skills. He will bring lot of success.
my daughter too lacked interest in writing initially.. but when gave some interesting stationary and different things to write she developed interest. I do not have much idea about the lack of sitting in one place, but if you can try to enroll for yoga/meditation it improves focus and concentration. Sometimes we may be dealing about a different issue and actual problem will be very simple.. All the best for your son's future. Best Regards.