I'm confused.
OP, you have said that your DS is unaware that he wasn't invited to the sleepover, and as such, he was not upset at all.
You have also said that in your opinion, other people's children should not have sleepovers where only a select few DC are invited to spare the feelings of others.
I am left wondering exactly whose feelings you think other parents DC should consider at their own birthday party.
Yours? Other parents? You have already said your DS is not upset in the slightest, so I'm assuming it is your feelings you believe should be taken into consideration.
The posts you have made where you have mentioned that your DS is a thoughtful kind boy, who really considers other people's feelings above and beyond his own, even on his birthday, and how proud you are that he wouldn't leave anyone else out, ever, even if that meant he had to sacrifice what he wanted on his birthday, being only one day a year to save the hurt feelings of others, well that has blown me away tbh.
You appear to be teaching your DS to consider other's feelings above and beyond his own.
Life will never ever be fair, and if you choose to raise your DS to do everything he can to consider other people throughout his life, even to his own disappointment, then imho, you are setting your DS up for disappointment. It is impossible to please everyone all of the time, and I don't think this is a good lesson to teach our DC to strive to attain this.
You are also in danger of raising your DS to be a people pleaser.
People pleasers are treated badly, taken the mickey out of, and not respected. They spend so much time and energy considering everyone else's feelings and still rarely please everyone.
Considering others feelings all the time, above and beyond your own feelings is a recipe for disaster.
Does it make you a nice person?
No, it makes you someone who defines themselves by others opinions and people pleases, and actually results in them being very unhappy IMVVVVHO.
Mind you, this is only my opinion. I am open minded enough to accept I might be totally wrong on this one.
Rant over.