Why does my 4yr old chew on things?

my 4yr old son chews on everything constantly. he chews things ranging from his blankets to plastic to paper and card board. his school has even tried giving him one of those special bracelets to chew on but its not working. i caught him today chewing on the cap to a bottle of water (he had chewed it in half completely flat). besides it being a little disturbing its starting to get dangerous. at times it seems like he is totally unaware that he is doing it. i have tried to deter it with edible but hot or sour tasting things and have had some luck but it has stopped working (how i broke the habit of fingers in his mouth). we have also explained to him that it is dangerous and he understands he can get hurt or sick from it. but he just wont stop about 15 Min's after the bottle cap i caught him chewing on part of a box to a toy, then at bed time it was a stuffed animal. can some one give me any ideas as to why he is doing this or how to get him to stop.