Hi my Zeus is only 4 months and he thinks everything is a chew toy so sometimes he nips on me or other people but I don't see any agression just mouthing maybe cause he's a puppy? how do I train him to stop? I try telling him NO! in a firm Voice and sometimes he does listen and sometimes he doesn't what else can I do since I know he'll be bigger when he gets older and I want to makes sure I curb the bad habits while I can still control him? thanks!