Visit your local holistic pet store and try a raw diet.  Obviously you will need to get a low-protein diet, but they should be able to lead you.  They can also advise on various supplements to help increase appetite.  I do not recommend commercial food toppers, as they usually contain some kind of protein and the idea here is to DECREASE protein, not add it unnecessarily.  They also tend to contain a lot of salt for flavor.  If you do not have a holistic pet store near you, visit holisticforpets.com.  The owner's name is Val and she is an absolute encyclopedia of dog nutrition and health.  You can email her through the site.  Good luck.