I think kids that didn't get to do much pretend play, like dressing up every day to play house or fireman or anything like that, are going to go through this at another time. They're going to dress up and be really goofy with it. Like they know it's not age appropriate but being totally a goof ball. But all kids need this stage. It's important for their development.
I remember when my grandson went through this. He was never in child care and didn't have any dress up stuff at home. He was encouraged to play computer games and watch TV and play outside. Normal kids stuff...
However when he'd come to my house where dress up boxes were abundant he'd put on dresses, dance costumes, fireman hats, cowboy hats and adult boots, and more. He was 10-13 during this time. The grand kids living with me were 3-4 and 6-8 and they played dress up all the time. Friends flocked to my house to play. The attraction, I'm sure, was dressing up the younger brother like a girl. He loved it too.
So I think if this seems to be the normal stuff, he wants to do pretend play with other stuff too then perhaps it's that stage that he has to go through. If he's seriously interested in girl clothes and dressing like a girl perhaps it will turn out he likes to cross dress. Some men do that and have no other gender or sexual identity crisis style issues.