I know this question both as a professional and otherwise. Children with PDD diagnosis can certainly stick to a line of thought and worry despite our best answers.
Take heart: What I have found is that honest, simple, consistent answers are best. Even those of greatest faith do not know exactly what lies beyond this life. Your son's question is valid, even if difficult to answer.
For every rational question that he answers, give a simple. consistent rational response. For some of my folks, even those very close to me, I would remind them of my answer; and encourage them to explore this question on their own. Those who express a personal faith can talk with clergy, etc.
Simple distraction isn't the answer as his question remains. Encourage his own exploration of this area as well. At 14, even with Asperger's, he should be able to do some of this...