Mr W (3.5y) is in with 4.5-5.5y in a curriculum based day care. He also has play dates with kids in the 5-9y age range. He gets along really well with them all.
In the case of his current school, he told me after his first day, "Dad, they can talk!" referring to his classmates.
Prior to that he was in a Montessori school with just one or two kids older than him in his room but often ended the day with a large group of mixed age kids.
In the latter case, he gravitated to the older kids due to the more complex nature of their games, and once the older kids realized he could hang, they accepted him. One kindergarten kid came up to me when Mr W was 2.5Y and told me, "He learns really fast!!"
Overall it has worked out in Mr W's case because he has been exposed to older kids from almost the very beginning. He is also very much an extrovert.
Mr W does get mad when he gets talked down to. We prep him when going to a new situation by reminding him to act like an adult, ie shake hands, introduce himself, and we encourage him to address people when they ask us questions about him. Once he breaks the ice then he gets included.
Perhaps you can work on social strategies with your DS on how to become part of the scene. IE, introduce himself, being aggressive in getting included, making one friend at a time, etc.
