Has a food allergy been ruled out? I was thinking of the salmon oil as some dogs are allergic to that. Also for the chewing, it could now be a habit and when the hair grows back it itches, maybe a type of spray to deter the chewing, like Natural Touch Chew Not for Dogs. It also has tea tree oil to promote healing. Also redirecting with a sound or using a toy anything to get his attention off of the chewing. You mentioned having also a homeopathic Dr., maybe there could be remedy to help treat the itching. I have used HomeoPet Skin & Itch drops for my Sting and they do seem to help. Also, have you thought of contacting the breeder in Germany and ask what food he was being fed as a pup and if they have any suggestions.
Sting Chance von Gaard AKC GSD 2/8/2006