This can symptoms of skin infection - Bald patches. How often you give bath to your dog? Do you allow him to seat on wet surface? Dogs that are given bath too frequently are more susceptible to skin problems. Dogs need daily grooming. Frequent water exposure is mother of all skin related problems.
To stop the infection further, please clean the infected area with Betadine and then apply Spectazole ointment all over the infected area. Do daily brushing. If your dog seats on wet surface, make sure you clean his body with a dry towel.
Also, I'd request you to visit a vet and ask for a good anti bacterial and fungal infection. You may have to shampoo him two weekly for few months. But, in general, keep the coat as dry as possible.