We are already pretty sure he will not be returning to this school next year, and have found some good potential options to replace it, but we don't want to pull him out now, when there are only a few more months left.
It might be only a few months but if your first choice for next year has an opening, I'd seriously consider switching right now. It sounds like a perfect opportunity to try out the new school to see if it's truly a good fit.
I like Montessori and I think it can be a good option for advanced learners but I wouldn't want to pay a lot of money for my child to do meaningless busywork.