E.S.
This sounds like high school, not adulthood. My advice would be the same thing I would tell my kids, move on and find some new friends, these people do not seem like the type that you would want as friends.
We do not have to be best friends with our neighbors. I'm not. I'm cordial, but that's about it. They don't hate me, I don't hate them, but for the most part they aren't the type of people that I want to be friends with. I have many other friends that I can drive to. My kids don't play with the neighbor kids despite them all being close in age. My kids have other friends and other activities to keep them busy. I'm too old to be bothered by neighborhood problems (my neighbors always seem to be in a "fight", I'm always the neutral party).
If your son seems to miss these neighborhood friends, enroll him in some classes to keep him busy and keep him from being bothered that he can't play with them. Take him to the park, forest preserves, etc. with just you or meet up with other kids from the class. Honestly, if this is the way the parents act, why would you even want your son to play with their children? Call on moms from school outside your neighborhood for play dates. Be cordial to the neighbors, but don't let it bother you that you aren't in their clique. Instead be happy that you aren't in their clique since it doesn't sound like a nice one!