I'm going to assume because your son expresses the thought that he's going to be an angel when he dies that he believes in an afterlife that is better than this corporal one.
My son, also six and in Kindergarten, has expressed opinions about how great it is going to be when he's dead and in heaven. He has been told that there's no pain in heaven and what is now hard will come easily. Our family is Christian, btw.
What I've told him is that God will NOT be happy to see him before his time. We've told him that God sent him here to complete a special assignment and if he does his time here, God will say, "Good job!"
This has helped tremendously. Our son has dreams for his future and has not said anything more about wishing to be in heaven.
Perhaps saying something similar will resonate with your son. He's been given a gift. Emphasize that. A gift to be used for good, and not to be wasted.
