LisaLisa, you know your child best of course, but you are speaking about a child with 4 older siblings, why would mixing with older children overwhelm him?
My sons preschool had divided the children according to age groups. 2-2 1/2 year olds, 2 1/2 to 3 year olds and 3 -4 year olds, where the oldest group received a more formal preschool learning. (They of course stayed in their groups as they progressed through the school) You may find that this is very common, especially if you find a montessorri preschool/nursery, which in any case is very focused on the individual child and their abilities and individual characters.