OMG!!! HE IS ADORABLE!!!!!!
AWWWWWWWWWWWWWWWWWWWWWWW WWW!!!!!
I wonder if it is a contact allergy? Something on the rugs or outside?
Maybe Vet will have a better opinion?
Yeah, that looks like "Puppy Pyoderma" (or puppy impentigo or puppy strangles or puppy rash..theres at least 5 or 6 common names for it) Its caused by a staph infection. Its very, very common in puppies, especially when they first start going outside and get little scratches etc. on the sensitife skin of their underbellies. You also see it in puppies who are being crate trained and urinate in the crate. This infection will start as a simple pustule, then the dog scratches it and it spreads to cause more pustules.
If its only 1 or 2 small pustules, it often is a self limiting infection. Give the puppy a bath every 3-5 days and keep it clean. In this case its progressed to an acutal pyoderma, which would be something I'd typically manage with chlorhexidine shampoo or other medicated antibacterial shampoo. Again bathe every 3 days or so. If you don't have access to this kind of shampoo, plain old betadine scrub (you can get it from any drugstore) will work. Just be carefule because betadine can dry puppies skin out really badly and it will stain white dogs a light brown color. Just as an FYI, in my experience, peroxide doesn't seem to work well at all. I wouldn't waste my time with it.
There are occasionally puppies that need to go on oral antibiotics for a severe case. I typically reserve those for puppies that are nonresponsive to topical therapy or that have pustules appearing in other places other than just on the belly or that seem to have very large pustules that are exremely painful.
Hope that helps.