Point out to your friend that his morals are those of a mere donkey: he does what's right only because he desires an eternal carrot, and avoids doing wrong only to evade the eternal stick.
Then remind him that you strive to do what's right simply for its own sake, and even in the absence of hope for an eternal reward. Remind him further that even though you fear neither judgement nor eternal punishment for committing wrongful acts, you also abjure doing that which is wrong or harmful - again, simply for the sake of NOT committing wrong.
Then ask him which of you demonstrates a man exercising his free will to lead a moral life purely for its own sake - and which is demonstrating merely the conditioned response of an unthinking beast that spends its entire life simply being driven along: one that remains incapable of distinguishing for itself, moral acts from immoral ones. Because instead of being taught critical thinking, it has grown up with its behaviour manipulated every waking moment: perpetually driven along its path by the dangling carrot and the brandished stick that make up Christianity's twin, false promises of eternal life.
Came here to say something like this but you said it better.
I like the donkey-carrot-stick analogy, going to steal that!