I am a high school teacher, and I've seen how kids with special needs are sometimes treated in a less-than-nice way. I'd encourage you to ease him into kilting, and make sure the first couple of times he wears the kilt that he's surrounded by his "support structure" of family and close friends who will brag on him and tell him how much they like it, and then maybe take him to a kilt night nearby to introduce him to other kilties in your area. If there's a Games close to you, it might be beneficial to take him there and get a stranger or clan member to explain some of the mechanics of kiltedness to him. (Teenagers often listen to strangers more than they listen to their parents.)
Above all, make sure the first few times he wears the kilt are positive experiences, and build from there.