Scheduling is often a real issue with subject acceleration in elementary unfortunately. We made it work but it was nice once we hit MS and left that behind.
1) ask them to tell you specifically what was missing.
2) potentially forget about this year but aim for next year and ask for a trial in 4th grade (assuming they know about the 3rd grade assessment). Tell them you will get a tutor for whatever 2nd grade concepts he's missing.
3) once you know what they think he's missing ask for a restest and brush him up on that (if it is a simple as exposure).
