Good luck with the doctor visit. The amount of weight lost is a gauge if he is not eating enough. Anything over 5% in a month of unintentional weight loss is considered cachexia, which can be self induced by anorexia, malnutrition, and other reasons, sometimes which there is no control of even with adequate nutrition. If the doctors think he is underweight, is not earring enough, there can be ways to increase the appetite, and gain weight, as Christine mentioned, and prescription medication, supplementation.