Haven't read all the responses yet. We're in the same boat. Ds is in a Montessori. We enrolled him as a Kindergartener this year, with the thought that he might very well do a 2nd year of kdg. His academics are strong, but he's on the immature side anyway and he had a very rough year (becoming a brother, changing schools). In the end, we have settled on moving him up to 1st grade, regardless of whether he's in a Montessori setting or not. If he's bored academically, then he'll act out I'm sure. Also, with peer and older models, he's likely to emulate their behavior, rather than sinking to the level of the 3 year olds in the Montessori primary class. Lots of people have advised me differently, and we struggled with the decision for a long time - finally coming to a decision last week. I feel good about it...but time will tell!
You could always do a trial year of kindy and see how it goes. (If possible, you may want to try it at a different school, so there's not the stigma of being "held back.")