D.O.
I do not have any personal experience with Montessori programs. However, my understanding of them is that (even in elementary grades) they are less structured than a traditional school environment. I would assume if it is a reputable program that they focus on socialization and early learning skills (as you mentioned) rather than an environment where he has to sit for extended periods of time. I would recommend talking to other parents whose children have gone through the same program. Perhaps they can give you some insight to put your mind at ease!