5 month old terrior/shepard mix with generalized red mange not going away?

I have a 5 month old puppy who is currently on medication for red mange and a secondary bacterial infection. He has been on it for a few months now and it just keeps getting worse. The vet said it is genetic because his mother had it and he has a weak immune system. I am wondering If there is anything else that can be done for him. He is miserable, and antibiotics with the baths and meds, don't seem to be helping. The last resort is the vet telling me to euthanize him because it will be a recurring problem. I rescued him from.the humane society and he is the sweetest dog ever.