Looks just like a heat rash, my Belle gets them , they are outside a lot, Bath the area with a shampoo, I use Molly Suds dog shampoo, its all I use now, it is fragrance & dye- free, rinses off well too.
Than I apply a herbal save of comfrey and aloe vera, You have to stay on top of it and to this procedure until it goes away or it wil turned into an affection. My three frequently get skin allergies, but never an infection. If your dog is scratching it a good deal, give him some benedryl, liquid childrens formula.DYE FREE TOO. , This will help tremendously, I let my Belle stay in the house when she has one of these hot patches. good luck. Dont waste your money on a huge vet bill, Keep the area clean and put the slave on it and a cone head so she/he wont like the stuff off.
I use Country comfort save, but anyone will work with the combination of comfrey and aloe vera.