Diabetes in dogs - Wikipedia, the free encyclopedia
Your dog has the canine equivalent of Type 1 Diabetes, so no, you won't be able to stop the insulin shots, or he'll die. However I would still recommend looking into a more biologically appropriate diet (I don't have a dog but maybe other pet owners can advise you). Most pet food is total crap, and grains are probably even less biologically appropriate for canines than they are for us.
In the meantime, though, if the dog is losing weight and doing well on his new diet, I wouldn't panic or anything. It's great that he's losing the weight- the way most people allow their animals to become obese is disgusting and I think it borders on abusive treatment.