I would make use of the situation to show your DS how best to deal with disappointment. I would explain that sometimes in life people do things we don't understand or can't explain and it can leave us feeling upset, angry and disappointed. That life just sometimes isn't fair and then explain to him how best to cope - that to retaliate would only bring yourself down to their level, to get mad at them would lose not only your dignity and respect but that they have their own choices and this is one they have made that you can deal with together. Explain how to rise above it and set an example on how to behave in an inclusive manner - it will also be a good lesson for him for the future if he ever leaves a friend out (remember how you felt that time, that is how X will be feeling now, was it a good feeling?) etc.
I don't know... Ds has just started school so he's out if the house most days (she's not yet at school) so they've seen less of each other a bit recently. And they've had their spats in the past, silly childish things saying "you're not my best friend, you're not coming to my party" but I didn't think it was anything more than just normal childish squabbles.
Our neighbours dd is a few months younger than my ds, they play a lot together, slip through a gap in the fence constantly and are always in each others houses. We're friendly with the parents too, our front doors are right next to each other, we have keys for each others houses etc.
I'm feeling a bit sad that they haven't invited our ds to their dds party. It's not a tiny party, about 15 children most if whom ds knows. I'm not sure what to do. Do I take it as a not very subtle hint and back right off? I don't think we're very pushy people, but literally the children are always popping between the houses and our doors are so close its hard to see how I can back off more.
I feel a bit sad for ds. He can be boisterous, I so hope it's not making him unpopular.