If underage kids drink at your party, you are liable under more things than the social host law. As a parent you have the duty to make sure that if you host a party that your son and his guests behave. If you can't accept that responsibility, you gotta play the mean mom and not allow him to have such a party.
Make it clear, no alcohol. If any kid shows up with alcohol or previously have consumed alcohol bar the child and call their parents. If it becomes clear that alcohol was snuck in, end the party and inform the other parents.
The NJ social host law is pretty limited, but that doesn't mean you won't incur civil liability, there are countless cases of people being successfully sued in excess of the social host rules. Further, since we are talking about minors here, the child endangerment statutes come into play as well.