Exclusionary behavior in a wealthy neighborhood

My 5 year old son recently started kindergarten and is struggling to make friends or even find kids to play with and I'm looking for insight and suggestions.  But first, a little background:

We rent a townhouse in a wealthy neighborhood with homes valued over $2 million. My son attended a Montessori preschool in the neighborhood from ages 2-3 but struggled to make friends despite being outgoing.  We thought it was a problem with his behavior, but when we witnessed first hand some of the unkind and exclusionary behavior from other kids, we decided to pull him out of the Montessori preschool and enroll him in a co-op, part-time, play-based preschool in our city's "downtown" area.

My son thrived at the play-based preschool. He quickly made many friends and despite occasional conflicts, by and large all the 20+ kids played together and got along. He also made new friends easily at two summer camps, and he’s always been open to playing with others, sharing toys, and initiating play.

Now, my son is in kindergarten at the neighborhood school with many kids from his old Montessori preschool and unfortunately, he's facing the same issues with exclusion. My son told us he has asked to play with multiple different kids, or just tried to join in and play with them, and has been told "I'm not your friend, I'm his friend." and "You can't play with us."  I have scheduled a playdate with a classmate/neighbor and was stood up and then ghosted.

In case it matters, the majority of families in our neighborhood are full-time, dual income, with many kids attending after school programs and being at school for 9 hours a day.  The downtown preschool he previously attended was predominantly single income homes with at least one parent (and often grandparents/family) very involved and invested in raising the kids, attending preschool part-time, and more racially diverse.  Our family falls into the latter category.

At this point I'm concerned with what appears to be an exclusionary culture among the neighborhood kids and wondering how best to handle it.