Start by expressing gratitude. Gratitude for the great lengths the kindergarten teacher has gone to serve his needs, and gratitude for his time to meet with you to discuss options, and the flexibility everyone at the school has shown to meet your family's needs. Express your understanding for the role of a teacher serving so many different needs in one classroom.

This sort of thing goes a long way, particularly since it is genuine.

Go in with questions: Ex: Here's a writing sample done on his own -- where would this place him relative to the first graders he'd be joining? What would be the plans going forward into next year if this is the arrangement, specifically, how will his needs be met in a 1st grade classroom next year after spending half a year in this arrangement?