He needs intervention with his weight loss, which is multifactal, possibly with an enternal feeding tube, medications, counseling, nutritionist, and speech, swallow therapist evaluation. Doctors get concerned when 10 percent of weight is lost unintentionally, and is considered cachexia or anorexia really, and he lost almost 15 percent. It is shown how one does during treatment, including maintaining weight, can effect prognosis, and too much weigt loss effects negatively, effecting healing, and bodies ability to fight cancer. As you probably know, and heard, he has one shot at this, but it's really up to him to fight or flight. Everyone needs help, and there are meds to increase the appetite, build muscle, reduce inflammation, improve mood, and easy ways to add extra calories to foods, plus other interventions mentioned. Thankfully, he has you by his side.

20 percent of cancer deaths are due to malnutrition from anorexia, cachexia, and cancer. Even 5% weight loss is a concern for some doctor's and clinician's. Here is a link for "Nutrition In Cancer Care PDQ" from the National Institute of Health, with plenty of information regarding thr proper need of nutrition with cancer, during treatment, recovery, therapy assistance, and others.

http://www.cancer.gov/cancertopics/pdq/supportivecare/nutrition/Patient/page1