My old grandfather just started giving importance to Islam today by me telling him. He grew up as a Muslim but he never/very rarely did prayers, fasts, etc. What should he do about his past negligence?
Your grandfather needs to repent to Allah for not observing prayer and other acts of worship like fasting, etc. In addition, he needs to make sure that he performs his obligatory daily prayers, writes a will indicating the number of years he missed prayers since the time he became a Muslim, and allocates some of his wealth to be used after his death to hire someone to pray the missed prayers on his behalf, as well as any missed fasts and pilgrimage if he was capable during his life and did not perform it. In addition, he needs to satisfy any religious dues that may be applicable to him such as khums.