I would not recommend using either on puppies that young when you don't know what the cause is. Using chemicals and antibiotics topically will probably only result in mama licking it off. Young pups do not have a developed immune system yet and using antibiotics on something that isn't a bacterial infection can inhibit development of their immune system and will do nothing to heal the rash. Mommas lick their pups in the genital and lower abdominal area to stimulate urination. This is a good thing since unweaned pups are not yet developed enough to evacuate waste on their own. For some puppies this can result in a mild puppy equivalent of diaper rash. I would just watch it and if it gets worse or seems to be bothering him, then take him to a vet for diagnosis before treating him.
Sometimes if they are not being cleaned well enough by there mother they can develope these little scabs from food, urine,or .You can clean his belly with some peroxide water it down.Then you can use some antibiotic cream or Neomycin and it should clear up in a few days.Good luck and Take Care.
Jody, I'm so sorry because I can't tell you what this is. If you take him to be checked by the vet, I think that is the best answer. It might be nothing serious, but best to be on the safe side with such a very young pup.