That's a very hurting thing to happen to a child. My daughter recently wasn't invited to the birthday party of the "popular" girl in class. Her father is a member of the school board in the district and she wears overly fancy clothes to school. The teachers have a tendency to favor her. She has an air of entitlement and she never speaks to my daughter out of class. If my daughter says "hi!" she just turns her head away.
However at least the invitations were not passed out in class. My daughter found out about it from the excitement of the other girls talking about the party, and I learned of it from another mother mentioning that her daughter would see my daughter at the party, then realized my daughter was shut out when I knew nothing about it.
What really irritates me is that the teacher this year, and last year are always partnering my daughter with this girl in activities and little jobs in the class, as well as sitting her near this girl, in the hopes that this girl's "popularity" will somehow rub off on my daughter. It really irritates me. This school is not a good match for my daughter and I really wish I could find something better.
Children can be very mean but unfortunately in your son's case the girl's parents might be leading her in the wrong direction. That's just very mean and bad manners to pass out invitations and leave someone out.
It's so sad. Maybe if the weather is getting nice you can take your son on a nice outing somewhere and do something that is his favorite, but don't talk about the party unless he wants to while you're out and about.
