Mysterious infection-several vets yet no cure

I have a 4 year old male JRT. 1 year ago he got an ear infection-runny pus-like discharge that caused itchiness. We have tried the following to get rid of this:

Cultures-revealed nothing
Ointments, creams and washes.
Medication (Baytril, I believe, and even an immune system suppressant)
Several vet visits
2nd and 3rd opinions from other vets-all baffled

No one seems to have any clue what is causing it. It has since spread to his eyes and mouth openings, with scab-like growths appearing there. Otherwise, he is very healthy and happy.

The next suggestion from our vet is to do allergy testing. I want the dog to get well, but it is getting expensive, and I am getting sceptical that anyone can cure this.

Has anyone ever heard of such a thing? I don't understand why this seems to be so mysterious.

I did some research, and discovered that Iams/Eukanuba changed formulas, and dogs have begun experiencing similar problems. The sad thing is that no one thinks of changing the food because they have given them the same food all their lives, and even spend more believing Iams/Eukanuba to be of higher quality than other foods.

We put him on hypo-allergenic food, and he seems to be recovering finally.