Pyoderma, or more commonly called "puppy pimples", is very common. It's no different than adolescent acne in humans. What kind of antibiotic are you giving? Is it a pill or topical cream? I think antibiotics is pretty extreme treatment of a harmless, and self resolving issue that you can treat with just cleaning the area with soap/shampoo a couple times a day.
Further, there is no need to change diet at this point, assuming you are already feeding a diet free of fillers and junk. As far as it being "hypoallergenic", I don't see any benefit in that as this isn't an immunodeficiency (allergy) issue.
Good luck.