What should I tell my son who has a fear of death like me? He is only six but seems to be afraid of the concept of death.
In order for you to help him, you will first have to conquer your own fears. "How can you say to your brother, 'Let me take the speck out of your eye,' and behold, the log is in your own eye? You hypocrite, first take the log out of your own eye, and then you will see clearly to take the speck out of your brother's eye" (Matthew 7:4-5).
Fear arises when we don't know what to expect. None of us have died before, so we don't have personal experience with what is awaiting us. Yet, for the Christian, we do know some things about what is awaiting us. "Therefore, since the children share in flesh and blood, He Himself likewise also partook of the same, that through death He might render powerless him who had the power of death, that is, the devil, and might free those who through fear of death were subject to slavery all their lives" (Hebrews 2:14-15). Barring the Lord's return, all of us are going to die (Hebrews 9:27), it is a simple fact of life. But we also know that we won't remain dead. "Do not marvel at this; for an hour is coming, in which all who are in the tombs will hear His voice, and will come forth; those who did the good deeds to a resurrection of life, those who committed the evil deeds to a resurrection of judgment" (John 5:28-29).
While we are waiting for the resurrection, the faithful of God rest in a place that is called Paradise (Luke 23:43). We also get to join with friends and family in Christ who have gone before us. In the Bible, when someone dies, it is said that he was gathered to his people (Genesis 25:8). Therefore, one thing to tell your son is "If both you and I serve God, then one day we be in heaven together permanently."
"There will no longer be any curse; and the throne of God and of the Lamb will be in it, and His bond-servants will serve Him; they will see His face, and His name will be on their foreheads. And there will no longer be any night; and they will not have need of the light of a lamp nor the light of the sun, because the Lord God will illumine them; and they will reign forever and ever" (Revelation 22:3-5).
The curse in Revelation 22:3 refers to the curse placed on Adam because of his sin. "Then to Adam He said, "Because you have listened to the voice of your wife, and have eaten from the tree about which I commanded you, saying, 'You shall not eat from it'; Cursed is the ground because of you; In toil you will eat of it All the days of your life. Both thorns and thistles it shall grow for you; And you will eat the plants of the field; By the sweat of your face You will eat bread, Till you return to the ground, Because from it you were taken; For you are dust, And to dust you shall return" " (Genesis 3:17-19). There is no sin in heaven so there will be no curse as a result of sin there (Revelation 21:27).
Of course, making the transition is a bit scary. But then so was childbirth the first time you faced it.
"For we know that if the earthly tent which is our house is torn down, we have a building from God, a house not made with hands, eternal in the heavens. For indeed in this house we groan, longing to be clothed with our dwelling from heaven, inasmuch as we, having put it on, will not be found naked. For indeed while we are in this tent, we groan, being burdened, because we do not want to be unclothed but to be clothed, so that what is mortal will be swallowed up by life. Now He who prepared us for this very purpose is God, who gave to us the Spirit as a pledge. Therefore, being always of good courage, and knowing that while we are at home in the body we are absent from the Lord -- for we walk by faith, not by sight -- we are of good courage, I say, and prefer rather to be absent from the body and to be at home with the Lord. Therefore we also have as our ambition, whether at home or absent, to be pleasing to Him" (II Corinthians 5:1-9).
The results make the change worth it.
When explaining these things to your son, keep your explanation short and simple. If he has more questions, he will ask them and you can give those short answers as well.