Very annoying!! I feel for you, my guy's party is Saturday as well and I just want him to have some fun with a few friends, too. Yet only 2 have RSVP'd out of 19 invites, and they are friends not at the school. Some have asked what he likes, a mom apologized yesterday for her daughter not making the party due to cheerleading competition (in preschool) this past Saturday, that she'd really wanted to come, I smiled and said the party is this Saturday and she said "Then I think she can come!" But no firm commitments, even though they have my cell number and email. And our school doesn't get involved in handing out invitations or reminders so I don't have help there.
I've seen this for years and truly don't expect people to RSVP anymore, though. What I always do is go big, I'm making a cake, cupcakes, chocolate suckers and food for an army. Reason being I want to have enough in the event they all show, we can eat leftovers and I can send some home with guests. Truthfully, we always get a crowd without the RSVP's so I'm guessing we're going to have at least 5 of his preschool friends, plus siblings, cousins, neighbors and family friends.
The only mom I know who got almost 100% feedback was the one who stated on the invite "Please RSVP by _______ so your child's place at the party can be reserved, per vendor policy." Only 2 friends didn't contact her, one going through a death in the family and one who was leaving the country on vacation at the time of the party.
Hope your son has a great party!!