You could always rent the full dress regalia----kilt, PC, etc...---then no worries about having to pay as much or keep a kilt that only marginally fits. Turnaround to get it, and exhange it if there is a fit problem, is pretty quick too. As long as your son is a fairly standard size, and you are not too stuck on a particular clan tartan.

Matt Newsome at Scottish Tartans Museum is guaranteed to do his best to make you happy. Were I to be renting I would contact him first. He is an advertiser here on the forum so should not be tough to find.