Why does my elderly Dad no longer believe in God?

Dad still prays but insists that there is no God. I don't know if he just wants somebody to prove or preach to him and convince him there is a God or what. I always tell him he is free to believe what he wants but that I still believe there is a God and always will. I am not a religious person and don't feel like I am the right person to preach or convince anybody that there is a God. I just know from my own experience that God has played a big part in my life. That is all I can tell my Dad. But he just goes on about how the bible is just a big lie or fairy tale made up by man. I know he is probably upset about our mother's (his wife's) death and that his health is failing where he needs to be cared for 24-7 and that might be a big part of his attitude. But I don't know what more to say other than he has a right to believe what he wants. And I will continue to believe in God and tell him the reasons why.