w/d is still the preferred diabetic diet for dogs. Unlike cats, carbohydrates do not affect glycemic control, but fiber does. Therefore, a high fiber diet is strongly recommended for diabetic dogs.
If your dog is underweight, I would concentrate less on the diet and more on glycemic control. When was your dogs last glucose curve and what were the results? If the diabetes is not properly controlled, then your dog is losing calories in an uncontrolled manner through its urine, and this is far more important than the calories your dog is eating.