Only God knows his heart, of course, and whether or not he was sincere when he prayed to receive Christ. But I seriously doubt if he was “faking it” (to use your term); when someone realizes they are about to die, they have little reason to put on an act.
Do you remember the two criminals who were crucified at the same time as Jesus? They were guilty of great crimes, and they knew they were about to die for them. One of them sneered at Jesus and refused to have anything to do with Him – but the other turned to Him in faith and asked for Jesus to remember him and save him. Jesus’ reply could not have been any clearer: “I tell you the truth, today you will be with me in paradise” (Luke 23:43). He had only hours to live – but Christ forgave Him when he turned to Him in faith.
Take comfort in your uncle’s commitment to Christ, even if it was at the end of his life. Jesus’ promise is for every person who puts their trust in Him: “He who believes in me will live, even though he dies” (John 11:25).
But don’t draw the wrong conclusion from your uncle’s example and decide that you can delay your decision for Christ until the last minute. We never know when our life will come to an end. The time to give your life to Christ is now.