You've had some great advice, but I have the feeling that much of it is from people whose children have bigger challenges than yours. I want to ask: what's stopping you assuming it's just asynchronous development? You say at the start that his writing is ahead, although not as ahead as his other skills; so, while it's good to make sure he doesn't get held back unnecessarily, in what sense does he have a fine motor disability? I'm not understanding.
Cricket, metis mentioned other areas that were showing as fine motor delays (difficulty with buttons/zippers etc), and I think metis mentioned a low score on the Beery VMI - this is a visual motor integration test used by neuropsychologist that is impacted by fine motor challenges. It is possible to have DCD (fine motor disability) and still score ahead of average on the tests metis has mentioned and still be ahead of the game...