A. Hair loss in dogs has many causes. I'd start with asking whether there is scratching to go along with it - that makes things like fleas a realistic concern. If not, he could have mites/mange, and ringworm should be considered if he's been exposed to other animals that might have it. Allergic disease (to food or something in the environment) needs to be considered if he's scratching as well. And hormonal diseases, such as hypothyroidism, may be a possibility.
Your account has been reactivated.