The creationist does not have to contradict his belief about the age of the earth to cooperate. He only needs to recognize that the way to get the best result given his belief is to exchange cooperation for cooperation, using common sense (saving 2 billion people given that the earth is young is better than saving 1 billion people given that the earth is young). Yes, understanding the prisoner's dilemma is harder than understanding poison is bad, but it is still a case where common sense should overcome a small bias, if there is one at all. You might have some work to convince the creationist that his choice does not need to reflect his belief, just as your choice to cooperate would not indicate that you actually believe the earth is young.

I would rather persuade him to cooperate but not to be superrational (allowing the outcome to be D/C) than persuade him to be superrational (forcing C/C), and I doubt the latter would be easier.

Why is he going to cooperate unless you offer to cooperate in return? Unless you actually convinced him to reject young earth creationism, he would see that as saving 0 people instead of 1 billion. Or do you intend to trick him into believing that you would cooperate? I don't think I could do that; I would have to be honest to be convincing.