Are his eyes closed a little? If they are then its a upper resporty infection. You should take him/her to the vet soon or else it could lead to death.
why does my newborn puppy sound like he has water in his nose when he breathes?What if a newborn puppy isn't breathing right?
HOw old is the puppy? and what do you mean by breathing from its mouth?10 week old puppy is having trouble breathing