Is he a Black, Red, Blue, Fawn? Photos? If he was totally Vet checked. What did your Vet say it could be? Maybe a second opinion?
Here are some other things to try that we use. Organic Extra Virgin Coconut Oil, Organic Egg, Vit E 400iu, Pure Olive Oil.
Have you tried adding some Probiotics in his food? Are you feeding also good treats? What were the other foods he was on? Maybe he is allergic to fish?
Has the Vet done a complete Thyroid panel??
Also I make my own coat spray. Found it in Doberman Digest
24oz spray bottle
Listerine (Yes the mouthwash)
Alpha Keri or CVS Shower & Bath Oil
Any Human Hair Conditioner I try and get an all Natural one
Filtered or Bottled Water
Kiwi Soft shoe brush
Microfiber cloth
Bottle mixture
Listerine 1in
Shower bath oil .5in
Hair Conditioner 2in
Add water to top
Spray coat with Filtered or Bottled Water
Wipe with cloth
Brush with soft brush
Wipe again with cloth
Spray with Coat Spray
Brush in with soft brush
Depending on the condition of coat. You can use daily to get coat in condition and then weekly to maintain