Help!

My 3-year old is chewing holes through his shirts. Something has got to be bothering him.  Has anyone gone through this before? The most effective thing so far has been making a spitting noise every time we see him chewing, which reminds him to spit out his shirt. But it's not working anymore. At first putting him in collared shirts that button up to his neck worked, but then he started lifting the bottom of his shirt and chewing there. I am throwing away 3-4 shirts a week. This has been going on for about a month now. I don't know what to do. We will consider and appreciate any tips you may have. Please, help!