Hi, I am taking care of my 8 yr. old Grand Yorkie Dog and I can bathe him and he looks great for 1 or 2 days, then he starts to get greasy, stringy and smells "like a dog", shhhh, don't tell him I called him a dog!

He thinks he is the BOSS... Could this be from age, shampoo, food, treats or what??? He has been to the vet with some skin issues and I have tried their suggested shampoo, but, one made him worse and the other seems not to help! He is a licker and a scratcher and he doesn't have any fleas. He scratches so hard sometimes that he makes red spots on his fragile skin. I will post a picture of this little man when I figure out how and get more time! Thanks!
Sparky's Grammy