4yr old chewing on shirts

I know its probably a nervous habit but im not sure how to go about stopping it. My son had an issue with chewing on his shirt sleeves in the past which i attributed to nerves from starting daycare/preschool. He stopped doing it for a long time but is now doing it again. Like chewing on the collar of his shirts so much he soaks the shirts and has to change it. He has actually worn the collars out on some of his shirts so much that they're ruined. There's nothing new going on that I'm aware of that would be causing the nerves. Any tips on this? We're going through sometimes multiple shirts in one day. Help!