Most Important: Reassurance that he is loved and there will be no abandment.
normally until 9 years of age, children do not have a realistic view of death--they typically think death is reversable: people can be alive again after death. It's by age 9 that they realize death's irreversability and universality (it happens to everyone).
So I see your child is a very precocious six year old, with a 9 yr old's cognition about death. Also there is a strong secure attachment between parents and the child.
Most psychologists stress that honesty is the best strategy when discussing death with children. Besides honesty in explaining about death to children, parents can make sure that their child understands that he or she is loved and there will be no abandonment.
Regardless of children's ages, adults should be sensitive and sympathetic, encouraging children to express their feelings and ideas.