I would ask him why he wants to wear a dress. Does he want to know what it feels like, does he want to wear girls’ clothes, or does he want attention?
If he wants to wear girls’ clothes, I would take him to the store, and buy him some jeans and a shirt with some pink or purple on it in the girls’ department, and tell him he can wear those to school, but that the reality is that a boy in a dress is going to get teased, and I don’t want him to get his feelings hurt. If he likes his girls’ clothes, and after a few days, feels “better” somehow because he is wearing them, we will get him some more, even girls pjs and underwear.
If he is adamant about wearing a dress, then I would tell him he can wear one around the house. If he likes the feel of non-pants, for some reason, we might visit a Util-a-kilt, or other boys’ wear that is non-pants (with underwear). He will still get teased, but possibly not to the same extent, and he will be able to defend his choice as menswear.
If he just wants to be different, for some reason (doubtful, but who knows?) maybe he just needs more input into choosing his own clothing, even if he chooses an oversized shirt, plaid pants, and two different socks.
If, after a little experimenting, it develops that he thinks he is a girl, and wants to present as a girl in more than just dress, but in hair, and with a girl’s name, I will take the idea very seriously. I am aware that people who are transgendered almost always knew from early childhood. I am also aware that some people are not traditionally gendered, and a few people go through a phase. I don’t want to take too big a step at once, and have to backtrack, but if he is transgendered, I want to make his homelife an oasis, because life in general is going to be tough.