Should I confront mother for not inviting my child?

[Mallorytimes]

My DS is 8, he is within a group of 5 boys who have been close since year 1. Generally all parties are either whole class or just the boys. One of these 5 friends had a party for just the 5 friends last year. This year I've found out the boy had a party but just invited 4 boys excluding my son. He had to find out in school, apparently they went to this boys house for computer games then all out for an activity. Ive always felt I've been friendly with the mum, no big friendship but always a hello on passing at the school run etc.
I feel bad for my son, he said he asked the boy why he didn't go and the boy said his mother said he wasn't allowed to invite him. We had a party for my son and invited all the boys in class but this boy didn't go as they said they were busy.
I feel a bit confused by it all and my son was quite upset.
Worth asking the mother or just leave it?

I wouldn't worry about the mum she obviously has a problem she's not your friend so I'd leave it let her feel awkward she's the one leading the way and your son will also learn by how you respond to her behaviour x

Yes I’ve got this going on at the moment. Mum had taken against my son and told hers they aren’t allowed to play together etc (having been very close).
This boy is horribly behaved but they are the type where it’s always someone else’s fault. It’s fine, whatever. I would organise something really fun for the four of them over summer tbh (as I did on the last day of school), but I’m a grudge holder.

Leave it but just invite 4 next time your son has a party.