I wouldn't use listerine...
.... it is definitely NOT for dogs. You may want to have a vet check his teeth...... I agree, it could be baby teeth that need to be removed. (Shelby & Seymour had some extracted when they were spayed and neutered) I also suggest you try a dog toothpaste or gel. They have tooth wipes also. We use Petzlife gel and spray.... it cleans teeth and also gives them a fresh smelling breath.