M.J.
I can see how this would be hurtful. No one wants to see their children excluded from activities deliberately.
My best advice would be to move on from this circle at school and try to get your son involved in other groups where he can make real friends. If these kids don't want him at their parties, they're not really friends (even though you may be with their moms). Does your son have any special interests? Getting him involved in classes outside of school can be a great way to connect with other kids who share the same interests and build friendships.
Also, keep in mind that it's not the quantity of friends that matters in the long run. I always had small circles of friends growing up -- sometimes just one at the time -- and I've kept in touch with most through adulthood (one, for 30+ years). I was bullied and chosen last for things and while it wasn't fun at the time, I turned out ok (I think!). :)
I think if you concentrate your energy on how to change the situation, rather than looking back and feeling bad about things you can't change, you can turn it around. Is it proper etiquette to reciprocate with invitations to parties? Sure, it's the ideal. But people just don't do that always and it's naturally disappointing when they don't. I do think if your son can form bonds with other kids over common interests -- not just being at the same school -- that might lead to new friendships and party invitations.
Just my initial thoughts. Sorry to hear things have been so hard lately, but hope it all turns around soon.