When they say "stay where he is" does that mean repeat K, or just go into 1st rather than skipping a grade? And how does his writing compare with his age-peers? You've got a slightly different situation if it's behind age-typical than if it's fine for his age but just doesn't match his reading.
What is he reading at school? Reading level not matching writing level is incredibly common and the school ought to be able to cope with giving him things that are interesting for him to read, while still working with him on writing at his level. That's really not so hard for them. Are they not doing that? Or is your concern that you think he needs a gradeskip, which they aren't willing to consider because of the writing issue? It's frustrating when writing is the only thing that makes a gradeskip impractical, and I hear that sometimes getting the child to type is a way round that, but that usually seems to be with slightly older children. (We're currently encouraging my DS to learn to type, but it's slow going.)
How's his maths? You say he has quantitative IQ in the 99% but you don't mention strengths there... is that too a lack of challenge issue?
