If your dog has lost it's appetite and is not eating enough food like they used too then you can give the dog some Vitamins and Supplements to increase and stimulate the dogs appetite.
You might also try changing the dogs dog food brand as it may just be that the dog doesn't like the current dog food anymore and wants something different to eat.
Try giving the dog some chicken and rice to see if they'll eat that better instead of it's regular dog food.
Another thing you could do to stimulate your dogs appetite is to try mixing some canned dog food and some warm chicken broth or other type of broth to the dry dog food as that can sometimes help the dog increase his or her appetite again.