Whenever a dog has lesions on only the back half (i.e. usually only uncluding the base of tail, legs, around the butt, and abdomen) I'm always worried about fleas or a flea allergy. Regardless, I would recommend year-round flea control with a reputable flea preventative such as Frontline Plus, Advantage/Advantix, or Vectra 3D. Any other flea product (Zodiac, Biospot, etc) does not work.
When your dog has areas of hair loss, he needs to be examined by a vet (even though its occurred before). Your vet should do skin scrapings to check for mites (or mange), skin impressions to look for bacteria or yeast organisms, and even a fungal culture for completions sake.
During this time, he should go on a course of systemic steroids, antibiotics (the skin is a huge barrier to bacterial organisms), and topical therapy (medicated shampoos, steroid sprays).
Even if he gets better, you should still continue the shampoos anywhere from once a week to three times a week. Steroid sprays are good as well since the weaker ones have no systemic effects.
Now there is a chance that the skin disease may be secondary to a metabolic disease such as Cushings (hyperadrenocorticism) or hypothyroidism
. A urine sample collected at home is a good screening test for Cushings (its called a urine cortisol: creatinine ratio) and a thyroid level can rule in or out hypothyroidism (if its low then you should do a full thyroid panel). If he really does have cataracts, then your vet can also check for diabetes
, although this is less likely to have effects on the skin.
If everything comes out normal, then I would recommend continuing the benadryl (1 mg/pound two to three times a day), omega-3-fatty acids, and shampoo therapy while maintaining flea prevention every month.
You can also consider a food trial, but typically food allergies are year-round. Still, it would be part of a comprehensive workup for allergies.
And if you have a dermatologist in the area and just want some answers, you could always have your primary vet refer you.
I really hope this helps! Please let me know if you have any more questions!