The Shirtless Wonder

I wonder if anyone can explain why my 4 year old has suddenly decided that he doesn't want to wear a shirt...ever?? The first thing he does when we get home is peel off his shirt. He walks around all night long without it. Then, after bath time, he absolutely refuses to put his pajama shirt on, he only wants the pants on. Isn't he a little too young to be acting like Al Bundy? I haven't been able to get a picture of this yet, but I hope to get one soon!