Have you ruled out any medical or dental problems that may be causing this? If so, We have always used plain old Ivory Bar Soap. Just take a bar right out of the package and rub where ever he is chewing and they really seem to not like the taste of it. We used this for years and years with our race horse's who were stalled and just bored most of the time. We tried other kinds of soap, this seems to be the one. Good Luck!