I think, as others have said, it may be time to impliment a budget. We had very little money when I was a kid.
I can remember my mom giving me and my brother $100 for new school clothes each fall. DB had to have name brand jeans and sneakers, so his was gone with one pair of jeans and one pair of sneakers (and maybe some underwear). He then worked odd jobs around the neighborhood to earn the money for the rest of the clothes he wanted. Me? I was happy to shop at discount stores (Ames, Kmart) and get a whole wardrobe of no name clothes with my money.
Make some rules (must follow school dress code, no vulgar words on shirts, must change clothes every day), give him a budget, and let him decide the rest.