My dog has a bald puss spot on his back?

It sounds like it may be an abscessed sore that ruptured. Puss is an indication of infection and antibiotics are the only thing that will clear up the infection. You can wash the area with some warm water and antibacterial soap and then try to clean it out as much as possible with the warm water and cloth. Put some neosporine on it and then take him to the vet for the antibiotics.