It’s very common and it’s not worth worrying about. Many boys simply like the feel of girls’ clothes. He ought to buy his own gear if he’s old enough to do so. If you live in a small town and shopping is too embarrassing try ebay. You may get some real bargains there.
Honestly, I was originally suppose to be a girl, but I came out a girl. My mom always put me in girls clothes and When I turned about 7, I got to go and have a play date with a girl from school. Well, by now my mom stopped dressing me in all the girls stuff, (which kinda irritated me). I was 10 and that same girl let me wear her clothes and I realized I felt more comfy in a skirt and tights, rather then jeans and socks. It is not a bad thing, it could mean he is gonna be a really feminine guy and he will get along with girls alot better or he is experimenting. If he is too young to buy clothes on his own, in most cases if you just let it runs it course most out grow it, but its not the end of the world if he doesn’t.