My husband has a lung disease. It’s not COPD, but the effects are similar (although his life expectancy is theoretically shorter).
As a previous poster asked, does your dad have a respiratory consultant and nurse? Is he assigned to a centre of excellence? Has he taken a pulmonary rehab course? If the answer to any or all of those is no, that is your dad’s next step(s).
Does he get dietary supplements? Simply fighting disease uses a phenomenal amount of calories, and it’s hard to eat, so the NHS will provide really high calorie soups and milkshakes, for example. He should ask for a referral to a dietician. My husband has managed to stop his precipitous weight loss, which was leaving him weak and prone to infection. In the meantime, as there may be a wait for a dietician appointment, I can recommend double cream poured over ice cream as a regular meal. Full fat milk; cream in his coffee; easy to swallow high calorie food like rice pudding, will all add calories without too much effort.
I know how hard it is for someone constantly breathless and coughing to exercise, but it’s vital. A fair bit of the weakness and weight loss will be from muscle wastage, and even small changes make a massive improvement. Again, a respiratory centre of excellence will have a physio who specialises in lung conditions and exercise, but any physio will be able to offer some advice.
Also, it’s easy to assume that any increased breathlessness, coughing, weight loss etc is a progression of the lung condition, but every change should be investigated, as it might actually be anything from heart changes to pneumonia, a chest infection to acid reflux, and that’s all treatable.