My 15 week old male is constantly nipping or biting our hands/arms. It hurts! We don't know what to do to correct it because everything we've tried seems to just make it worse. We will grab the scruff of his neck. Or grab his tongue/bottom jaw with our thumb and hold it for a few seconds. But then he just barks at us and tries again. He's stubborn. Anyone else with this issue that has curbed it? SOS. Please help!