Question for VET PHARMACY

My one year old German shepherd dog, in the last 2 months has developed a skin disease. all the belly skin is red and has lost all heir, the rest of the body looks still fine apart some small spot which has the same problem. He has a double coat fur. I'm combing and bathing him regularly weekly but is not improving. What problem is this? What should I do? What medicine should I give or apply to resolve the problem? Thanks for your help.