You have not mentioned the exact disease of the lung for which he has undergone pneumonectomy
. All his symptoms are because of surgery as now he has to make do with one lung. You should continue oxygen therapy
for as long as possible. He can even be put on non invasive ventilation, esp during sleep which will give rest to his breathing muscles. Avoid infection of any kind as it will worsen his condition. He may be vaccinated with influenza
and pneumonia vaccine
.Take care of his nutrition and over a period of time he will get used to one lung.