The first thing you should do is take the dog to the vet so that they can get them on antibiotics to clear up the existing infection. After that, I would look into switching his diet to a higher quality brand dog food.
Keep his place dry and clean. The yeast infection it make him to lick himself and than the legs or parts of body will be wet and the infection will go on and on. Sometimes it helps shaving the dog parts infected to keep it dry.