I took some advice from Becky at http://organizingmadefun.blogspot.ca and re-folded his t-shirts so he can see what he owns. Maybe this will help him see something else to wear.
My daughter on the other hand, knows darn well she has too many clothes (thanks to very generous friend's hand-me-downs) but her challenge is to find the one pair of pants she wants without destroying her entire dresser. I can't guess how many times I've gone into her room and found almost everything she owns dumped on the floor! She hangs all of her shirts, but has 4 drawers full of bottoms - leggings, sport pants, skirts and pants. Let's hope this method will the do the trick for her too. In her case, I just rolled them up and put them in some plastic shoe boxes we had laying around not being used.