Most likely, it is due to stress. But it could also be a medical problem. This website has a list of problems that have hair loss as a symptom, the most common are allergies, stress, and hyperthyroidism - although I doubt it's hyperthyroidism since he's overweight. Most likely stress
http://www.peteducation.com/article....&articleid=195
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~Magnifique Yorkies~
Purchasing from backyard breeders, pet shops, and puppymills perpetuates the suffering of other dogs.
Educate yourself and buy from reputable breeders or rescue.