I am sorry to hear about all the problems your dog is having. My name is Rebecca. I am a veterinarian and will do my best to help.
If his underlying problem is allergies, then treating for allergies might help his skin to heal. The new allergy drug Apoquel has helped many dogs. About 80% of allergic dogs respond to this prescription medication, and you know if it is going to help them in a matter of days. Unfortunately, about 20% of dogs are not helped by Apoquel.
There is also the possibility that this is more than allergies, such as an autoimmune skin disease such as pemphigus or lupus. If a skin biopsy has not bee done, that might be a good idea.
The supplements that help some dogs with allergies are nettles, quercetin, anti oxidants, fish oil, and vitamin E. As with the Apoquel, what works for one dog does not help the next.
Bathing in oatmeal shampoo may help with the odor and the itching.
Sometimes when there is purplish dark skin discoloration it is a sign of secondary yeast infection. The underlying problem is still allergies, but treating for yeast can help with the smell. Shampoos containing ketoconazole and Triz EDTa may help. In severe cases, oral anti yeast medication like fluconazole may help.
I would consider seeing a veterinary dermatologist, and consider doing skin biopsies.
Let me know what other questions I can answer.