food allergy dermatitis, or environmental allergies, mites, and other ectoparasites, ringworm (a skin
fungal infection), bacterial skin
infection (pyoderma), autoimmune skin
disease (lupus or pemfigus), endocrine disorders (unlikely at your dog's age) and others.
Skin problems often occur as combination problems. Allergies could be the primary problem but there could be a secondary staph pyoderma (bacterial skin disease), or there could be mites such as scabies, or demodex.