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.
For context, my DS-now-9 started school at 4y10m writing no more than his name and that reluctantly (though reading chapter books etc.) He struggled through the first few years at school - I used to describe his writing as "age appropriate on a good day" - and he isn't grade skipped partly for this reason. But at the recent parents' evening teacher after teacher told me his writing was no longer a problem. (A couple did still say his writing is slow, and it's still a relative weakness, but it's no longer limiting what he can show he can do as it did.)
Is your DS interested in playing an instrument? If so I think I'd encourage that. I've heard that different kinds of hand strength don't always transfer, e.g. the strength you get from piano may not help with writing, but I'm not convinced, and in any case music has many other benefits, e.g. perseverance practice, social opportunities and a way to meet others of very different ages.
