One question: do the different grades have math at the same time of the day? Subject acceleration in math is such an easy solution that it seems like a no brainer for them to just put him in the 2nd grade class for math and then have him return to K for the rest of the day if that is feasible given the timing of math in the various grades. Of course, the assessment of his math skills that you mention should be done first.

Do you trust that they'll accurately assess your ds's math achievement and has he had any ability testing (IQ) done yet? The only problem I've ever seen with math subject acceleration seems to happen much later (around Algebra) with kids who were accelerated solely based on high achievement and whose ability doesn't lend toward them continuing on the trajectory of acceleration later down the line.

Study Strategies for Accelerated Learners