Unfortunately for your doctor, there are these things called facts, which can quite readily prove him wrong. If you are not on insulin now, and can achieve normal blood sugars through diet and/or medication, it means your pancreas still has insulin producing cells left. The only reason you would need to inject insulin is if something happened to these. The reason you are classed as a type 2 diabetic is that some of your insulin producing cells have been destroyed. This is most likely because you have become insulin resistant. For a lot of type 2s, the cycle goes like this: A high carbohydrate diet is followed. This causes weight gain, and high insulin production. With a lot of insulin in the bloodstream, large amounts of glucose is stored, and becomes fat. Also, the body starts to become resistant to insulin. Many type 2 diabetics actually produce more insulin than a non diabetic, but they cannot properly utilise it because of their resistance. This puts a lot of strain on the insulin producing beta cells in the pancreas, and over time they start to permanently burn out, leading to type 2 diabetes.
If you change your diet (and the fact you have lost so much weight already shows this is what you have done), and increase your exercise, your insulin sensitivity will increase dramatically. Medications such as metformin can also achieve this. This will take the strain off of your pancreas. If your blood sugars are within normal range, providing your lifestyle is healthy, it suggests you still have enough insulin producing cells left to manage well. So my question to your GP would be this: If your body can produce the insulin you need, and you have removed the process of insulin cell destruction, why would you ever need to inject it?
