Thank you for your comment gardenlady!
The lung specialist opted for drawing fluid from the lung to see if that gives us answers before attempting a lung biopsy. The verdict is in: The fluid contained lung cancer cells. This is my husband's 4th primary tumor in about 6 years. He had unrelated pancreatic and throat cancers at same time, followed by skin cancer a couple of years later, and now the lungs. Apparently, he is one of those unlucky people who have no resistance against cancer. According to his oncologist (due to liver, kidney and other health issues) chemo is out of the question this time. Radiation is also out on account of cancer already in the plural fluid, meaning it has spread already. A possible treatment he has to be tested for is targeted therapy. He'll be having a blood test Monday looking for genetic mutations that could help with targeting the cancer cells specifically with fewer side effects than systemic chemo infusion. If that doesn't pan out, he may be out of options. From everything I'm reading online, prognosis is pretty grim: A few months.
I want to thank you all for your support, suggestions, and shared experiences. I will probably be seeking support specific to lung cancer now. Best wishes for your continued health journeys!