My 3year old large mix breed dog has been diagnosed with undetermined skin allergies. Our vet pushed us into giving him prednazone which didn't help him at all and has since been angry with us for giving it to him because he is now overweight due to the water it makes him retain. His skin has gotten so much worse and has turned black with red sores aroung the edge of his belly and underarms. We've changed his food several times, even trying vegetarian and raw food. We've had him on several different antibiotics and shampoos and creams including for yeast infections but nothing is working.
Please if someone knows what to do let me know, we can't stand to see him suffer like this.:(

Since you used several different antibiotics part of the problem could be that it has stripped your dog of good bacteria so you need to give him some probiotics.
Probiotics, What Are They? Do our dogs NEED Them? (http://www.thewholedog.org/id24.html)

Natur's Way digestive enzymes and probiotics for dogs and cats | Beneficial bacteria for healthy digestion and a strong immune system. (http://www.preciouspets.org/natursway/)

Also since his skin is so bad you should try giving him some flax seed oil to see if that will help get his skin back to healthy

Digestive enzymes would probably be helpful too.

My dog had little sores it looked like little red pimples on her belly, around her back legs and in the creases of where her arms would be. I used otc dog hydrocortisone for her stomcah and legs, that seemed to bring her some relief.

She also had a terrible odor even, an hour after being bathed. On her back there was a huge spot that was turning dark that was also red and cracked.
The vet gave her a steroid shot , he said the effects would last for 3 weeks and a medicine that would fight the yeast infection that he said was on her back... (that was the reason for the odor from her skin)
She stopped most of the itching, almost immediately and the infection cleared up in a week or so.
I felt so sorry for her that I hovered over her and kept looking at her skin to make sure there were no fleas and I think I made things worse on her by acting like that.

I can get the name of the medicine when the vet opens if you would like... he said this started from a flea allergy. I know how bad you feel for your dog. I hope you find something that works for him