
I really feel for you. My last GSD sounded like your boy, at the age of 9 months we considered euthanising her, but we did eventually find a way to manage her condition.
As you are probably aware allergies can relate to diet, atopy (something in the environment) or contact. In my girl's case she was affected by all three, yet with careful management we managed to keep her virtually itch free.
A few questions now. Firsty is he insured. If so I would ask to be referred to a Dermatologist. I don't know where you are, but Sue Paterson (rutlandreferals.com) was who we saw and is a leader in the field of Dermatology. Your vet will be able to direct you to a specialist in your area. Dermatologist can carry out patch testing, which would involve him being shaved and his skin being exposed to apprx 50 common allergens. Once the allergen/s have been identified, it is possible to start immunotherapy treatment to try and desensitise him to the allergens.
Feeding - Have you had any tests carried out to see if he is allergic to certain proteins/grains, etc? Allergy screening for food intolerences can be done by means of a simple blood test. To be honest, as much as I believe in raw feeding, with a dog like your boy, I think it is really important to find out if he is intolerant to certain things. Once you have the result, assuming he has positive results, your vet will be able to start him on a prescription diet that excludes all the things he can't have, and you can start a food trial to see if things improve. At this stage he MUST not be given any extras/titbits other than the prescription food. I know you have fed a food from the vets, but did they explain about totally excluding all other foods from his diet, during the trial period?
Finding out what the allergens are can be difficult, but once you know you can do a lot to minimise your boy's exposure but you need the help of a good vet and dermatologist. From the way you describe your boy I think you need the help of a good veterinary professional to help and guide you through the process and with their help I am sure you will be able to manage his symptoms successfully.
My girl was described as the most allergic dog she had ever come across, yet with careful dietary management, shampooing to control secondary infections and the occasional used of the dreaded steroids during major flare ups (maybe 2 or 3 times a years for a couple of days), my girl led a very happy life.
I know it's a bit of a long rambling post, but if you would like a chat, pm me you number and I will give you a ring.