Gosh. What is normal at seven is exactly these smaller parties of ten or so. It is also completely normal that children will talk about their parties at school, including to people who aren't invited. They get excited about birthdays and parties. 6/7yos are not adults with fully developed social graces.
Most DC cope with this adequately at this age, though not always. Mostly they understand that each child is more friendly with some people than others, so in many cases they wouldn't expect to be invited, or even want to go. There are though plenty of 'borderline' cases where kids do feel left out.
It is also completely normal for parents to go through an amount of tricky negotiation, when deciding with their DC who will be invited. Parents are more aware of the longer-term implications of short-term choices and that 6 and 7yo friendships can be very unstable, with someone in favour one week, not the next. My experience is that it is tricky but, so long as you're not doing anything blatantly horrible, like inviting every boy in the class bar one, you just have to do your best, get on with it and, stick with your decision - parenting in a nutshell really.
My observation is that you appear to expect far too much of neurotypical DC. You seem to be putting them on a pedestal of adult social expectation, expecting them to be everything your DS is not. They have far more more in common with him than you think. They're all 6/7yos, all with their own social struggles and disappointments.
You also seem to expect their parents to have undergone a training course in dealing with the specifc needs of your child. That is a really odd expectation. the revelation that he is among kids of parents who might not have ever had to read about ASD and neurotypical and non-neurotypical and who don't coach their kids to be sensitive to the feelings of others makes me sick. How on earth is that 'a revelation'?!
Yes, parents should and do teach their children to be kind and, children sometimes but not always remember. Again, they are all 6/7yos, they all get excited and want to talk about their parties. I think it would be quite unhealthy to expect them not to. It would actually mean teaching them to be sneaky, manipulative, tell white lies and form cliques - excluding uninvited DC from everyday play, so they don't become part of a 'party' conversation. Be careful what you wish for.
Much better to teach them to be open, honest, naturally excited and say 'sorry I could only invite ten people'.
You know this is a massive overreaction and won't help: inside I am absolutely distraught and spent the afternoon partly in tears and apologising to my DS for all the times I failed as a mother.
No-one is being 'bitchy' here. That's a horrible thing for you to say about parents and children who are behaving completely normally and being in no way unkind.
You need to be really careful not to project your negativity and twisted interpretations onto your own interactions with people, as well as your son's. You risk becoming the bitchy one, by seeing and responding in kind to 'bitchiness' where there is none.