I'm not getting the big deal here, but I don't really understand why the gender divide is so important to begin with, I admit. If your son is worried about her seeing him changing, maybe the best choice would be to change far away from the entrance?
In the female changeroom at my gym, it's not very well labelled, so men are always walking in there by accident. If I'm naked I just say "Oh, this is ladies" and they apologize and leave. Anyone can be a predator; gender doesn't really matter. So I think this isn't as horrifyingly damaging as you're making it out to be.