To be upset for DS (excluded by friends)

[Fruitflylady]

DS (13) found out on Saturday evening that 8 of his friends were all at a sleepover he’d not been invited too. Only 2 others of his school friend group had also not been invited. He has since been told it was a joint birthday celebration for three of them, one of whom, let’s call him A, has been his best friend since they were 5 years old.
He messaged another friend, let’s call this one B, during the evening while the party was going on to ask where he was (no reply) and later heard this child B on a live Instagram video saying my DS ‘has found out about the sleepover’, which suggests they’d all talked about him not being invited at some point before.
How can I help him move on from this? He’s very upset as he now thinks this best friend, or indeed any of his friends, don’t like him as much as he thought. I am good friends with both boys’ mums....is it worth mentioning to them or will this cause more trouble for DS?

My dc are adults now but they were on the receiving end of this crap a few times in both primary and high school. It's very hurtful but I don't believe there's any point in asking other mums about it.
What I tried to say to myself and my dc is that sometimes they would inadvertently be the ones on the inside and would they really appreciate being berated for being invited?
Friendship ebbs and flows through school. Some dc are very focused on being perceived as "popular" and will happily ditch long term friends in favour of the in crowd.
It sucks but there's very little you can do about it except be supportive and sympathetic when it does.

I’m not yet into the sleepover age, but 8 Kids sounds like where I’d top out and there’s got to be a line somewhere. Given other parents reply I don’t think your son has been “left out” it’s just one of those things. Credit to you and the other parent on talking it out tidily, this sort of thing could have escalated but feels like you’ve both dealt with it well.