I am understanding that his appetite is just fine, but he is still losing weight. The vet will be able to determine the underlying health issue - it could be a pancreatic problem (do his stools look and smell like undigested food?) or maybe diabetes (does he eat and drink more than he should?) It could be something as simple as a parasite too (usually involves loose stools and a lack of appetite).
On pancreatic problem:
My dog has been losing weight for sometime without losing appetite
The veterinarian suggests it could be a pancreatic insufficiency, what is that? Can it be treated? Pancreas is a vital organ that produces hormones, like insulin and enzymes, to help digestion in the thin intestine. In a case of pancreatic insufficiency, there is an inadequate production of enzymes, leading to an incomplete discomposure of food to be absorbed by the body. As a result, food is defecated almost without digestion, with a clear color, not solid, and a disgusting odor. As the dog does not get the appropriate nutrition from his food, he starts losing weight for some time. This insufficiency can be confirmed through clinical exams. Treatment consists in giving the dog sufficient enzymes through food, normally as capsules or powder, to compensate this deficiency. It can also be helpful to add ox's gizzards (pancreas) to the diet, this increases the proteins level in relation with that of the carbon hydrates and greases. Unfortunately, though this supplement can improve the digestive process, affected dogs do not gain weight, and remain thin. It's not difficult to take care of a dog suffering of a pancreatic insufficiency.
On diabetes:
http://www.petdiabetes.org/diabetes_concepts.htm