rac, after reading your follow-up comments, I can understand how you would be apprehensive about committing to a year of studies with that teacher. I likely would be.
Can I ask some more questions to help you structure the situation?
Assessing probability of securing a grade 1 (or later) space:
-Do you have a good sense of intake levels for the school for the early elementary years?
-Do you know who the usual teachers for the next few grade levels are and whether they'd be a better match for your son?
-Is there a possibility of deferring your son's enrollment until next year?
School awareness of your son's needs & relationships
-What does the principal of the school think of your son's test results?
-Do you have a relationship that would allow you to express some of these thoughts informally?
-Is the school open to skipping or accelerated placement in the likely event that you cover beyond K curriculum?