I've heard this a couple of times and I still can't believe somebody could wear their kilt backwards and not notice it was different from
EVERY OTHER KILT WEARER IN THE WORLD
Show him a picture of your favorite pipe band.
Take him to your nearest Highland Games.
Ask him if he's ever seen anyone else wearing the kilt.
When he first got it and put it on, did he check with someone to see how it looked?
Did he see a picture of it before he ordered it?
Obviously you shouldn't say any of this to him.
Like everyone else is saying. Be nice, but definitley tell him...before he runs into someone like me