Chewing shirts...

It's a new habit that Ben seems to be forming and I really want to de-form. He get the neckline of his shirt and puts it in his mouth, holding it there with his teeth. My first thought is that this is stretching the neckline of his shirts and they are going to all start looking daggy. I have just had the though then, that maybe he has transferred some of his thumb sucking time to this shirt chewing thing. Don't know why and don't really know what to do about it except to tell him to get it out of his mouth when I see him doing it.