L.
I also think that it's incredibly rude and insensitive for people to not show up to a party - that has also been my experience, and that of my friends recently. I think people are just SO busy and tired, AND just way too casual about responding to invitations and actually coming - even when they RSVP that they are coming! Well, I could sound off on that for a long time, but I do have some suggestions for future parties, to avoid that problem. It seems to me that the only people who really do show up are those that you (your son in this case) are actually really involved with (and those with good manners - rarer and rarer these days). It also sounds like your son is feeling a little lonely. With those things in mind, my suggestion is that you get him involved in some extra-curricular activities - either with the school or outside of it (sports leagues, church, classes of some kind - music or art, for instance). That way he can make more friends and they are much more likely to actually show up to a party or event. Also, it does seem to help to have a more "formal" invitation. Ie - if the invitation is super-casual (like just giving a blanket invitation to the class and not sending out individual invitations), peoples' responses seem to be too. If you mail out invitations with their names on it, etc., asking for an RSVP, you are more likely to get a better response. (Many people STILL don't respond to those, but you will get a higher percentage than with a casual invitation). Most people do like to feel specifically invited and appreciate it when you go to the trouble of filling out and mailing invitations.
I hope this helps and that your son has a really great birthday! Blessings to you and your family.