My concern really isn’t that he’s wearing her undies, but if the kids at school catch on. He may just like the feel of them. I’m not sure I would tell him to stop, but maybe he could just wear them at home instead of to school or in public? Kids are so curious and other kids can be soooo mean.
So I would say,”I know you like the feel of them and they feel more comfortable, but my concern isn’t that you like wearing them, but what might happen if others at school were to find out. Kids can be mean and cruel. Maybe you could just wear them around here?” There are truely some kids and even adults who could be incredibly mean. That is all that worries me.