I got to thinking, is it possible your dog chewed on a houseplant? Some houseplants are poisonous. Or some kind of fluid in the garage? Or insecticide or pest poison of some kind? Also, raisins, onions, alcohol, chocolate, (and even sugar) are big no no's- is it possible your dog got into any one of those things? It would be helpful if you could figure out a cause.
Have you given your dog any pet meds or vaccines, or used flea products, including flea shampoo, lately? The rash could be a reaction to any one of those toxins/poisons.
Hard to say what the root cause is. Could be environmental- exposure to cleaning product, pesticide on lawn. Most allergies are food related, and allergies can develop spontaneously- so perhaps you could try changing to a food with limited ingredients, (less ingredients, less things to be allergic to). I know the **** Van Patten brand (at Petco) has a limited ingredients line of food for allergy dogs. Also "California Natural, Pure & Simple" (lamb & rice) is supposed to be a good one for sensive dogs. I have a friend whose dog is chronically allegic to everything, and she gives him nothing but boiled chicken and soft boiled eggs- he's allegic to everything else.
You might try giving him a pro-biotic daily, and a daily immune booster to help get his system back in order from whatever has caused this upset.
I have a dog with super sensitive skin- he breaks out in a rash every few months for unknown reasons, and so I keep a bottle of "GenOne Spray" by VetOne on hand- it works like a miracle, and the rash is gone within 2 days. You might ask your Vet for a bottle- 'might just do the trick for your dog, as well. (nothing else works for my dog)
Could it be something he has come into contact with either in your garden (even lawn fertilizer or something) ...or while out walking?
Some plants can cause rashes, but usually with itching as well. Nettles is an obvious one. My dog once ran through nettles and had a similar rash. Also I have heard that Poison Ivy in dogs is not common, but can sometimes happen. And there are many other plants, both wild, and in gardens, which COULD cause rashes.
Was blood work done and if so what where the results? That rash could be the result of either allergies or an infection.
If the medications are not working yet and it is an infection it could take a few more days.
Is he scratching it? One of my dogs used to get heat rash that looked like that but it was localized to one of his hind legs.