It could be that his concern is based not on you wearing the kilt, but more about what other's reactions about you would be. When I first started wearing kilts, my wife was like, whatever, but her mom was really concerned. After talking to her, turns out she was more worried about what other people would think of me being kilted. After seeing me kilted for a while, and seeing that I don't mind what other people think, and actually after seeing that most people don't react one way or the other, she got used to it, and now thinks it is kind of cool.