بسم الله الرحمن الرحيم
(Fatwa: 154/115/L=01/1435)
Calculate the salah which your grandfather could not offer during the said period and pay equal to one sadaqah al-fitr i.e. 1.633 kg wheat or its price for each salah to the poor and needy ones. It should be noted that you will have to pay only for fardh and wajib salah, thus there will be six salah per day.
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband
