Dear Mr Leonard, GSD is one of those breed who is prone to skin infections and so taking care of them is of prime priority. As you mentioned about his food, first of all plz get rid of beef and chicken for few months and try feeding him egg white and boiled rice. Moreover, buy a tea tree oil and apply gently to his coat every morning. Next go to a good vet to rule out any bacterial or parasitic infestation. If vet diagnosed positively then he will give some short of sub cutaneous injection and topical solution to apply. Follow things and then let me know. Dont worry he will be perfectly fine.
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Dr.Sri Krishna Mishra