When my now 6 year-old daughter was 4, she would constantly suck and chew on her sleeves and collar. It bothered me mostly because she'd do it so much there would be a hole in her sleeve. Her clothes looked awful since the material was ragged and stretched out. In addition to the messed up clothes, she developed a rash on her chin and around her mouth. I read everything I could to find suggestions on what to do to stop this behavior, but I ended up just waiting it out. After about 6 months, she simply stopped. MOstly, I think she got sick of the irritating rash and having cold, wet fabric rubbing against her skin. Give it time. If your son doesn't stop by the time he gets to 1st grade, maybe ask your pediatrician for advice.