The world is full of teachable moments and the fact that one slipped by should not distress you. You were just not prepared to teach at that moment and you will be better prepared next time.
As far as the boy wearing the dress, a lot of boys put on dresses and don't become CDers. it's not as though we catch CDing from frilly clothes, after all! Chances are, he'll have is fun and have nothing left from it other than memories of a good time and an appreciation for what women go through to look good.
Now he might put on a dress and realize that there's something special there, but that something existed before the dress came into the picture. Such an experience might well save him from decades of vague self-loathing before he finally comes to grips with his "inordinate interest" in feminine things.
Unfortunately the mainstream population is blissfully ignorant of TG issues and we are one of the the last minority groups that is still subject to blatant ridicule and hatred. That doesn't mean that I sit idly by while people express hateful things about the community. It is indeed possible to make it known that such behavior is not acceptable without outing oneself.
Questioning a parenting decision does require a light touch. One might ask what the parent fears in allowing their son to dress up as a female character but raising a direct objection would not be productive.
It might be helpful to ask a couple of questions. Has he dressed up as a ghost? A zombie? A vampire? All of these are manifestations of characters living after dying. If the mother is worried about wearing a dress costume causing gender issues wouldn't dressing as ghoulish characters lead to far worse outcomes?
