One side of lung dark means that there is some opacity there, it may be one or more lobes or it may be whole lung and his severity of his symptoms will vary depending on the extent of involvement. There are many causes for this kind of finding commonest being pneumonia
. There are many causes of pneumonia as well including the C word. Since he is already consulting doctors, they will do all relevant investigation to arrive at a diagnosis. My advice is at this stage you follow your doctor`s advice and hope for the best.