pleural thickening may have various causes (mild parynchymal lung disease, chronic empyema , tuberculosis, malignancy etc .) pt may be asymptomatic or may have severe respiratory distress. so please tell me the chief complaints of your father, a brief history of the sickness. In case of pleural thickening, work up is necessary. so please do a contrast enhance CT thorex, pulmonary function test & general blood investigations. CT scan is a safe investigation,  it have no such risk that u had mentioned in ur question. After that u can consult a good thoracic surgeon/ physician for particular advice. u may send the investigations report to me also.