My son is 13 and has not really gone through puberty yet either. He also dressed like a girl for Halloween, and went around with his 2 best friends, both 15 year old girls. I doubt he would change in front of the girls (but now I realize I'm gonna ask him!) because of his lack of body hair. I would be concerned about friends (especially opposite sex friends) not giving each other privacy because I think that being watched while you're getting dressed can be uncomfortable.
But I'm writing here because my son's interest in girl stuff is not just because of Halloween. He wears eye liner every day, which drives my husband nuts. He is trying for an "emo" look (not Elmo!) with long hair in front of his eyes and very tight jeans. He spends the nights at the girls' houses and often comes home wearing their clothes.
I don't think my son is gay. I think he is waiting for his hormones to arrive, and until then it is easier for him to pull off the androgynous look than a masculine look. He is also loving the close, chatty friendship he has with these girls; they all hug each other good bye constantly. He has a crush on one...it is easy to imagine her getting a boyfriend her own age, and then he will be sad.
I've talked to him about how it would be difficult if he was labeled gay by other boys. At the school he goes to, it doesn't seem like a big deal, but at the regular high school, it probably would be. But I got a sense from him that being gay has a very cool, fashionable mystique to it with these kids.