We were extremely lucky at that age since our Montessori early childhood and elementary classes shared a large space back then. I know the teachers observed that my son gravitated towards the older kids and really enjoyed them. Does your school have an elementary class? If so, would they be willing for him to participate with them during some activities? Even if he only spent recess or lunch with them, he might find the peers he's missing. Or perhaps he could join the elementary when they are reading. Just some ideas that I've seen in action successfully at our school- ymmv.
