My favorite doctor, Dr. Jennifer Miller has published a paper on the phases of appetite development in individuals with PWS. Check it out.
http://www.ncbi.nlm.nih.gov/pubmed/21465655
Dr. A. Sharma, Chair of the Canadian Obesity Network, has blogged about it here:
http://www.drsharma.ca/obesity-eating-behaviour-in-prader-willi-syndrome.html