Encourage him to write! Sounds like he has a gift for it. (Might help with the developmental issues too.)

This sounds flippant, but isn't: Rather than the mother typing out his stories, perhaps he could start doing it himself - not because of work-saving but because of the spontaneous line from imagination to medium, which can lose out if the flow is interrupted by having to involve a third party. I'm putting it clumsily, but many writers say that they type all their own work simply for this very reason, and a lot of creative material appears during the re-types as they go along.

I hope something comes of his storytelling. Fantasy certainly didn't do J K Rowling any harm!