the shirt chewing may have become a habit, like nail biting, finger sucking, lip sucking, etc.
Ask him why he does it , check his mouth that there are no sore patches and Ulcers, how are his teeth doing is there tooth ache, ask him if his mouth or anywhere hurts,At 3.5 years you'll get an answer,Dont make a big deal of it,sometimes kids that could do with more attention do negative things that will get them attention, lots of Games, reading, Fun,get Dad involved to do guy things like play ball etc