For starters:

http://www.albanach.org/facts.htm

http://kilts.albanach.org/tweeds.html

http://kilts.albanach.org/faq.html

All of these articles do a good job of pointing out that a lot of what we "know" about kilts, about what a proper kilt is, about what a kilt should be worn with, are the most arrant of nonsense.

Further, should he continue to believe that "if it's not tartan it's a skirt" I suggest that he tell these chaps that: http://www.royalirishrangers.co.uk/uniform.html

I find the persistence of myths about kilts to be a fascinating reminder of the very human desire for certainty, when the reality is probably a lot messier than we can even imagine.