Who among us hasn't dragged her feet every now and then on a self-improvement project? We all get in ruts, and sometimes, it helps to have a partner to facilitate the first step, especially if you don't even realize that's what's happening ("Come to the gym with me just this once, I have no one to talk to!"). And since his primary partner is you, this would be your job.
Make it insanely easy for him to get rid of the clothing you think he shouldn't be wearing anymore. This might mean cleaning out your own closet and making a pile for Goodwill, and asking him to contribute. And then helping him decide what to contribute.
As spots in his side of the closet start opening up, just begging to be filled with new, stylish, appropriate pieces, you'll face a whole new challenge: the "S" word…