my little boy is afraid to turn 5

So my 4 year old who will be 5 in a week was just watching tv and someone was getting the happy birthday song sung to them. So i told my son carter that in a week that would be him making a wish and blowing out his candles. He turned to me with tears in his eyes and just said "mom im scared" i asked of what and he said "of getting old and that one day ill die" now he has experienced a few deaths of close loved ones in his short 4 years all from cancer "they were sick" and this isnt the first time he has said something like this and i no he has the thought you get old and pass away in his head and i dont want to lie yet i dont want to scare him i only was able to respond with your a big boy but your still very little. My little boy is afraid to turn 5 how do i change his mentality. what do i say he made me feel completely speechless. help! picture of my silly guy!