Puppy pyoderma or impetigo given the location is a possibility. Its a staph infection of the skin.
http://www.peteducation.com/article.cfm?cls=2&cat=1592&articleid=878
And I've found a chlorhexiderm shampoo can help a lot. They have prescription grade shampoo and also a lower concentration one that can be found at pet supply stores.
When a pup is in tough condition like this the last place that gets nutrition is the skin and that leaves the skin open for infection. So some antibacterial care should help.
Its also possible this is a yeast infection and that would take different meds to resolve.
If you don't have weekend access to a shampoo consider using Betadine iodine solution on the spots. That does a good job of knocking down many bacterial and fungal infections and could help. For topical itch control you can also spray the areas with a Lidocaine spray but the steroid you are using should be helping a lot with itching.
You might along with everything else you are doing consider giving him a daily multivitamin mineral supplement and some omega 3&6 fatty acid supplements just to give him a good nutritional basis for skin repair. Vita Stat, or Theralin are two of the many multivitamin supplements and Derm Caps or Linatone are often used as fatty acid supplements.
Hope this helps you!
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