It's interesting that ours are pretty much opposites in some ways. DS8 can write with ease (though not always neatly...), but he gets lost in the ether if he has to do things in his head. I wonder if your son's stronger working memory is the issue there. Hmmm...
Have you tried having him record his thoughts and stories, and then transcribe them? As he gets older, then, he could transcribe them himself. I found this strategy very useful when writing particularly sticky parts of my first (unpublished) novel, actually. It lets you wander around in your thoughts without losing the thread.
Digital recorders are quite cheap now ($30-$50 buys you a very serviceable one), and fit in the palm of your hand. So even if he's hard on it, it's not a giant investment. They are small enough to lose, so maybe have him "borrow" it when he wants to record something, if you decide to go that route.
If nothing else, it's a good way to get those amazing stories he's creating recorded for posterity. My cheapy recorder came with computer software. I have some cute things recorded!
And no, I do not work for a company that makes or sells recorders.
