I would have his thyroid levels tested as hypothyroidism is pretty common in dogs and can cause obesity very easily. It could also be some other type of hormone deficiency or even too much of a hormone.
There is a newly-approved weight loss medication for dogs available now. I don't know much about it, but it might be looking into
http://dog-care.suite101.com/article..._drug_for_dogs
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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.