I am sure you realize that your father is grieving. Losing a spouse is so difficult because the loss is so great and also because it makes the surviving spouse face his own mortality. Sometimes, people deal with it by shutting down and letting go of everything, including their own health and well-being.
By allowing himself to decline, your Dad may be shortening his own life, and that is a selfish act. Prayer and loving reason will help him to see what he is doing to himself and to you. Also, as time passes and the wounds begin to heal, he will see things more clearly. He needs to feel God’s love like never before. Let God love him through you.
Pray for him constantly. If he is not a believer, tell him about God’s love and pray that he begins to experience it for himself. If he is a believer already, then remind him about the love of Christ. Remind him that, “we know that if the earthly tent we live in is destroyed, we have a building from God, an eternal house in heaven” (2 Cor. 5:1). Encourage him to find his hope and fulfillment through Jesus Christ.