Would he be going to a different class for math? Do you know who the receiving teacher would be? Do they seem open to working with your dc? Honestly if the k teacher is cool with it and the 2nd grade teacher is open to trying it, I think you are home free and should hopefully not need much interaction with the principal. I doubt the teacher would be suggesting this if they weren't pretty certain it was going to be okay.
DD did the "big math skip" in k. Content wise it went fine. The K teacher did all the leg work to make it happen, which was awesome. However, the 2/3 teacher seemed less than pleased with the situation - she retired at the end of the year so maybe it wasn't really about dd. DD did not have access to a desk, chair, or her own supplies in the older math class for several months but somehow she persevered. The school only had one class for k/1 and one for 2/3 so its not like there was a better choice of teacher in our case at that time.
DD's was able to keep her math acceleration when she changed schools. (Okay, really the first year in the new school she didn't really do math except once a week while the new school figured out what to do.) The kids in her math class are used to her being younger and help her stay organized, walk her to class, deliver her homework at the end of the day -- really I don't think she needs to be cosseted so much but the school is pleased that the older students have this opportunity for mentoring so if that makes it work for them, fine whatever works. Reactions from the parents have been more mixed. The parents in the older class have been fine to us because dd so clearly belongs there. Her age mate parents I think are relieved that dd is not in regular math anymore because she would intimate and/or outshine their kids. The kids and parents in the grade in between have been more problematic but it tends to be less obvious after the beginning of the year.
DD recently skipped in math again as part of a group ready to move onto middle school math. So far so good. There have been issues surrounding getting all the homework done when it is "boring", timed tests, and knowing how to fill out a scan tron form.
I know things around the holidays can alter school schedules. Perhaps right after Thanksgiving would be a good time. Otherwise if its going to be another week or two to get everything organized, you could start immediately after the Christmas/New Years break. This might be a nice option if you are concerned at all about other parents talking about your child's math skip. Either way, the more consistent the math class can be the better.
Two things my dd always wants to know about her math class:
-where will she sit? (does she have her own place)
-what will happen if either her age or math class has a substitute, field trip, or change in schedule? These kinds of changes seem to necessitate dd advocating for herself with strangers.
I hope my ramble is sort of helpful.
