i may not be much help but what kind of shampoo ARE you using? My dog had a skin issue after we bought him. It didnt start right away but he was so dry and always itching, I noticed some greyish spots on his thigh and top of his head. He had a skin graph done (you should maybe) and the 1st vet said nothing
. I went to my current vet months later when his hair became very thin and he just did NOT look healthy. They also did a skin graph and told me right away that he has Dermodex. They said that there were mites that never were killed off and pretty much burned his skin. They gave him cream that was applied. They also gave us a shampoo to use
Relief Shampoo: Pet Shampoo, Shampoo for Cats & Dogs - 1800PetMeds (similar)
Epi Soothe Shampoo For Cats, Dogs & Horses: Pet Shampoo - 1800PetMeds (simialr)
I dont have the shampoos with me but these look similar to what we were perscribed.
Im not saying that yours has what ours had but there are some shampoos that will help. Also a skin scraping may be able to make sure he dosnt have some kind of skin disease.
Have you tried giving him childrens benadryl for his allergies?
Also adding flax seed to his food made a HUGE difference for ours!