You have a very difficult job! I wanted to give you the comfort of this passage from the Orthodox Christian web site(see below) as well as some ideas as to what to say. Your son needs to understand that sickness as well as health are part of life. Death is part of life, and explain to him that this life is temporary and explain your own religious views. You didn't mention what they were, so I'm explaining some things from a Christian viewpoint. It's important to tell him that God loves mankind and that the baby is so sick that he will be going to be with God who will make all things better including his brother, and that when we die, we will go to be with him. We will see him again in heaven. This is very sad - even Jesus wept at Lazarus' funeral. So it's OK to cry. I agree with the person down at the bottom that said be honest about what's going on and allow him to be part of the baby's parting, and go to the funeral. I remember my sister was never told about my grandfather's passing - my parents either overlooked her or thought she was too young, but she didn't find out until weeks after the funeral. She was devasted, even though she was only 6 at the time. I was 11, but I was glad I was included in everything that happened, even though I was very sad and wept bitterly at the funeral. But don't ever let a child think that God wanted that person to die early to be with him. That can make the child hate God. Death is a way that the merciful God dealt with sin, because if we all never died this planet would become a hell on earth, because sin would stay with us for all eternity, and this planet would get worse and worse and there would be no end to that worse and worse.
Hope, the Conqueror Over Death
HOPE is the cardinal virtue and attitude of an enlightened Christian which gives a divine scope and purpose to his life. Hope is not a wishful and uncertain sentiment as it is in regards to the worldly things of life. In the Message of Jesus Christ hope possesses the golden link which connects the human aspiration with the Divine Truths. It is a hope for salvation; a hope that Almighty God is looking upon us with fatherly love; a hope which strengthens the human will to accept anything and everything in life as God's Will.
The source of deep comfort is the hope of God's Will in our everyday life and especially in times of death. The divine comfort is a gift, it is the only way to pacify our mind and heart. The Christian should be prepared to face the events of life which includes that of the death of our beloved person. The source of comfort is the Christian hope. The Apostle Peter calls it "a lively hope"
(1 Peter 1:13), "The hope that is in you" (Ibid., 3:15).
How can a Christian nourish his hope? How can he renew the courage he needs for life and death? The Apostle Paul placed hope along with love and faith; love, as a faithful service to our fellowman, and faith, as a loving devotion to God. The unshakable ground on which the hope of a Christian depends is the faith in a living God which is expressed on constant prayers, a Christian life and philanthropic attitudes towards society at large. With such a hope the Christian should withstand sadness as well as happiness, because "the hope we have (is) as an anchor of the soul, both sure and steadfast" (Heb. 6:19).
*The quotations in the text without any references are taken from the rich hymnology of the Orthodox Cburch, or are obvious Bible quotations.
I pray that God's comfort and mercy be upon you and your family and that through this extremely difficult trial you would all be drawn closer to God.