Here here! Just say something like: Well, what is he expected to learn this year and does he already know it? Has he already mastered what he needs to in order to be promoted.
Short, simple, reasonable. Then, they'll give you what you need in terms of how they view a gradeskip for him, what they think he already knows and maybe even expose their philosophy on gradeskipping in general.
Dottie gives great advice. Be non-committal. It is reasonable to say: Let me think that over and I will get back to you.