Do you have any test results (like the annual assessments/standardized testing) you could use with the school?

I agree with George about asking for a meeting with the principal to plan for next year. You might bring results from Mathnasium with you to show the level at which he's working and start with asking what they might propose to meet his educational needs (that's a buzzword phrase)?

Sometimes single subject acceleration is an option (i.e., going to a higher grade just for math).