I don't think I read anything about how the dog is reacting to this rash? Is he itchy, scratching it?
I encountered a flea allergy with our 1 cat - not until she was 10! She is indoors only, so never got flea prevention as our Shih tzu was on stuff. But the last month (Oct.) of her life (16 years) we did not have her on any flea meds. Well she got fleas & then they jumped on the cat. It was a good month plus, till we all (vet included) figured out exactly WHAT she had.
It was indeed a flea allergy! Very rash & sore like. During this time she was going nuts! It was so itchy for her.
So thus why I asked how HE is reacting. If he is going nuts with itchiness, my vote too is for flea allergy!