Montessori worked very well for our ODS (PG based on testing). Montessori programs, like people, are highly variable, so your results may vary. Our YDS (likely gifted, only 3 so we are watching and waiting...) didn't thrive as much in a (different than older son's) Montessori but is doing very well in a mixed age (3-6) preschool. I do wonder how he will do next year when many of the older kids (his normal playmates) leave.
The fact he says it is boring would be something I would watch. That is more concerning to me than the single age range. For the record, our ODS gets along pretty well with age mates (if you leave out more than usual indignation when they are illogical), but when he was stuck with age-range content in first grade, that was a problem. A big problem.
