It sounds as if it could simply be allergies. Has he been scratching frequently? Allergies can cause dry spots on a dogs skin, hot spots is there common termonology, and it is allergy season. My pooch would get them along with other itchy and swollen symptoms. Now he gets zyrtec everyday and the spots and other symptoms have all but disappeared. Check the skin in his ears, and around his belly. If you notice red blotches, I would suspect allergies. Consult a reputable vet to be sure. Good luck.
More information would be helpful.....My first thought is "Ringworm". You might google and see if ringworm fits what your seeing.......Karla