Oh, yes. Someone recently Googled the search term "how to tell a guy how to dress" and subsequently found the DressThatMan.com web site.
The Dr. Phil inside us is concerned because, what you are Googling seems to be more of a communication problem with your guy with a mere sideline of taste in fashion. If this is a new relationship, it may not be wise to "tell him how to dress" at all, and... if it is an established relationship, you've probably already told him (more than once) and he's thrown away all of the dingy underclothes by now and felt like that was a sufficient upgrade. Sorry, but because of that supreme sacrifice he doesn't hear the complaints now. Not even when replying, "yes, honey... whatever you say."
In either case - a more gentle form of coercion can then be utilized. Like secretly measuring his pants and ordering some really super tight, pimp daddy deluxe 1970's polyester flares for him yourself. Yes. With your own money. Along with a color coordinated 70's disco shirt to bring the ensemble together. Yes.
Once you place your order, a few days later the package will arrive. Set the stage for the reveal. Perhaps a home cooked meal, romantic music, then right before dessert... surprise! I've got a gift for you... and give him the package.
If he says NO, then you can say no, too. That's all the incentive he needs.
Or, so we've heard.