Talking with the administrators seems to be the best coarse of action.

Emphasize that the kilt is a cultural emblem and that your son wants to honor his Scottish heritage. Mention that the kilt is considered to be formal wear and has been worn by many as such (Prince Charles,Sir Sean Connery,Ewan MacGregor ...), and that your son will be wearing black tie to the event (dressed more formally than most of the students attending ).

Maybe bring some pictures of Scottish formal dress so they get a better idea of what you are talking about.

Anyway, I hope this has helped. Best of luck to you and your son from a local O.C. student. =)