Technical term is ''tensional outlet'' , which includes things like rocking, nail biting, sucking, chewing, nose picking, etc. A couple of discussions by parents on the chewing/sucking on clothing here:
Giving him something to fidget with when he is tense rather than chewing might help or things to chew, crunchy carrots, etc might help when he is somewhere he can utilize these things. You can also do a search for "tensional outlet" online and find articles relating to it. Some say to ignore it. Occasionally it can be a sign of other underlying problems, but generally the quick read I did on it said it is a way for normal kids to deal with stress.