Maybe dermatitis with a secondary infection from scratching. I hope others come along and give opinions. I know when my puppy was younger he got dermatitis around the nose area, but it just looked like a bald patch. He also had a bump in the mouth. Vet said some puppies have a lower immunity and dermatitis was common in young dogs.
Hw old is your dog? I think that might help others. Do you think he could have gotten bit by any insects?