Please keep in mind, these are my "opinions" and I am not a vet.
There could be numerous things going on.
- Did you change dog shampoo, laundry detergent, dog bedding. Anything that is coming in contact with that part of his body?
2)How long are you allowing him to be on a food before you change him? Sometimes it takes months to see a difference in their skin. I have noticed dog food allergies also cause ear infections, have you noticed anything like that?
Does he have fleas? It only takes one to cause a severe reaction in allergic dogs.
I thought Revolution was an oral med? If so, I doubt it would be the problem.
5)If this rash has been going on for a while, he needs to see a vet. They may need to put him on steroids to get it under control.