بسم الله الرحمن الرحيم
You or any other family member or your father himself should read Surah Fatiha (full Surah) 41 times daily any time (it is better if it is read before Fajr salah) with durood 11-11 times before and after and blow on water and give that water to your father to drink. May Allah give your father perfect health. Amen!
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.