Do a CBC ESR, X ray Chest, Pulmonary Function Test (PFT), Serum IgE, Fasting and post lunch sugars and show the reports to your physician who would advise you appropriate medicines. From your description of the symptoms it appears to be an allergic cough.

Hi Is your dad sleep cycle is regular does he have any addictions. Some times lack of sleep and excessive heat will also give dry cough particularly agravates in night due to bronchoconstriction. Give him luke warm water continously min 3 litres per day give him herbal decotions made up with cinnamom,dry ginger,tulsi and honey. avoid sweets and milk products Try L Hist Mont Once In night for a week