Birthday parties

My son's 7th birthday party is in a few days. We've only had 4 kids RSVP. When his classmates had birthday parties this year, it seemed like easily a dozen kids were in attendance (everyone in the class was invited to my son's, and to theirs). It's too late to cancel and plan something else. There's no obvious reason for kids not to want to come to my son's party - his teachers say he plays with everyone and anyone. Maybe it's just unfortunate timing. How can I save this party?