ANSWER:

His children and grandchildren must keep reminding him with wisdom, good manners and knowledge. However, if his memory has totally deteriorated and he is classified as insane and cannot differentiate between day and night then he’s not accountable and it doesn’t matter if he does not pray. 

My maternal grandfather who is 95 years old doesn’t offer Salah anymore.its been almost 1.5 years. He has always prayed all his Salah including tahajjud all his life but now he just ignores the call to prayer even after reminding. He is weak, prostate problems and weak memory. What can we do so that his last days are his best days.