Is the 1st grade montessori as well? Will they let him continue where he left off? If it were my kid, I'd want him to continue to 1st. My only concern would be that there will be repetitive info in 1st grade as well. I don't think it would be too beneficial to essentially keep an HG kid back a grade where he will be among the oldest. He probably will find a better fit in 1st (or 2nd...) Also depends on the receiving teacher. If you think there may be better differentiation in kindy, then maybe that is a good option.
My son has a Jan. birthday, so I can relate. We did not have a Montessori type option, and we ended up sending him to kinder with age mates and then having him skip 1st grade. That plus a move to an HG school midyear worked for last year.
