The teeth seem to be fine and the gums normal, and, however, the mouth still stinks. Look at his lips. These layers of skin - especially marked in Boxers, Saint Bernards, and other similar dog breeds – can swell up.The inflammation can result in eczema in the lips which is unpleasant as it is hard to treat. Anyway, the veterinarian has special adherent creams, and a general additional treatment will alleviate the situation. Sad to say, it is a slow process. In some severe or persistent cases the only solution is to operate, which is no big deal as all the doctor does is sew the lips together.