If you recently started using a different shampoo your dog could be allergic to it. Try using a hyper allergenic shampoo. But some dogs are allergic to certain foods so if you just started a new diet you can switch it back to his old diet. It could be mange though, but mange can be treated by cleaning daily. Just think about anything that could have changed in your home that is frequently used by your dog. Other than that, only a trip to the vet can help.
