Hello. I'm sorry your grandfather is ill. Poor appetite
and weight loss are common among patients with COPD. The mechanism(s) for this isn't always clear, but several studies show that individuals with COPD exhibit hormonal
derangements (growth hormone
resistance, for example) and elevated levels of inflammatory markers
in their bloodstream. Recent research suggests that altered responses to leptin may also play a role. (Leptin is a hormone produced by fat cells; when leptin levels fall, appetite usually increases. In COPD patients, this "signal" is somehow turned off.) These physiologic aberrations suppress appetite and, when coupled with the increased metabolic needs of most COPD patients, contribute to rapid weight loss
Certainly, all possible causes of your grandfather's poor appetite should be investigated. When all is said and done, though, he may simply be suffering from the "anorexia-cachexia syndrome" that is common among patients with chronic diseases.
I hope that answers your question, and I hope things go well for your grandfather.