The dog needs to have a physical first. You need to make sure that his weight loss isn't due to any medical issues. If there is something plaguing the dog, throwing food at it will only put weight on the dog, but not solve the long term issue. Please schedule an appointment with the vet, have the dog tested for worms and any other parasites that may lead to this condition. Throwing food at a wormy or sick dog is an uphill battle, better to get a clean bill of health first, because then any food provided will be absorbed and used most efficiently.
-Kim