Thanks for using Petco Pet Education Center, formerly Petcoach! This may be due to a focal infection. You can try to use a medicated shampoo containing chlorhexidine based shampoo and bathe him twice weekly for about 1 week. If he does not get better, have him evaluated by your veterinarian for further assessment and treatment. Alternatively, this could be secondary to allergies. Hopefully it is just an uncomplicated superficial infection. Good luck!