I also think you handled this one well. The only thing I would add, especially in light of the already quoted forum rule, you might want to talk to the parents at some point away from the child and explain the difference between wearing a kilt and [that thing we are not discussing]. The forum rule here stems from the fact that the two are very different things, and I would hate to see someone associate the two as the same and thus think of the kilt as a more acceptable version of something they find otherwise unacceptable. I do not know much about Autism, but from what I read the issue may be simply one of not understanding social norms and not one of gender at all. However I would not presume to be able to figure this out, especially based on a forum post.