There is a lot of freedom today in how people dress. I hope this freedom will keep increasing. I look forward to the time when men can wear dresses without it being a big deal, just like women can wear pants. So I would certainly let a boy wear a dress if he wanted to -- although, depending on the social context, I might have to warn him that he might get weird reactions.
I'm curious about this question, btw. Does it come from an experience of yours?