I would not be concerned. Is there any harm being done? Some kids like to chew on things. This kid prefers shirts. Some day he'll tire of it, I expect.

Maybe you could also try some blowing exercises to give more lip and muscle control. So blowing balloons, blowing out candles, blowing the recorder, sucking out of a drin
k bottle are all useful. Our OT recommended this and helped my daughter tremendously