How can comfort your cousin whose father got cancer?
Pray for God’s healing. Everything is in God’s control.
James 5:13 Is anyone among you in trouble? Let them pray. Is anyone happy? Let them sing songs of praise. 14 Is anyone among you sick? Let them call the elders of the church to pray over them and anoint them with oil in the name of the Lord.
Sorry to hear that. Pray for him. You may try to help him have faith in God, in God everything is possible. Jesus is the true healer. 
tell them the promise of God’s healing through the scriptures
tell him to believe in God, God is the healer
I think sometimes it’s comforting to say nothing at all.First we can accompany him to do something, or walk together;Then, when he wants to speak, we can expresses our understanding and comfort to him .Finally, invite him to pray together and pass on God’s promise and grace to him.