Could be a skin irritation and your doggie may be allergic to his shampoo, etc. Get some neosporin and put on them.
This is just a guess--but one of my dogs had canine acne. They formed around his muzzle when he was young (maybe one or two?) and were pus filled. The vet said they weren't a big deal. (These were fairly small--no bigger than 1/4" across, although he did have a number of them.)
It can bacterial infection called mastitis. This condition should be treated after diagnosis from vets. For details, visit this link.