What denomination are you? That will guide recommendations.
As a Catholic, I would tell my son that life is an opportunity to show your love and gratitude to God and share His love in the world with others.
ETA: this point might be a nice segue into a discussion of a line from the Lord's Prayer, "Thy kingdom come on Earth as it is in heaven". You could discuss what it means to make Earth more "heavenly" through prayer, alms giving, and fasting.
ETA 2: as a Catholic, I'd jump on the opportunity to teach the significance of Mass as a tangible, earthly experience of heaven and God as the sacrifice at Calvary. Scott Hahn has a lovely title, "The Lamb's Supper", which discusses the topic succinctly.
HTH!
