Having worked in a center setting for numerous years as both a Infant/Toddler/Preschool teacher and assistant director, I can assure you there are many factors that will often determine how and when children move up to the next age group. Age and readiness are certainly factors, as well as space availability (as someone mentioned) and the child's schedule. Perhaps the other child is part time and was able to fill the opening they had in the toddler room (???).
I would definitely keep open communication with both your child's current teachers on his progress, as well as with the director. Even if there is not room to accomodate him full day yet, perhaps there are times during the day when number are a bit lower and would allow him time to begin mingling with his friends-to-be.
Another option of course, is to look for home-based care where he would be interacting with a wider age range since family childcare are not limited to caring for just one age. Good luck to you and your little man!