Unless he figures you're not an idiot and you already know that, in which case it's better for him to have a rule that says "always one-box on Newcomb-like problems whenever the payoff for doing so exceeds n times the payoff for failed two-boxing" where n is a number (probably between 1.1 and 100) that represents the payment differences. Obviously, if he's playing against something with no ability to predict his actions (e.g. a brick) he's going to two-box no matter what. But a human with theory of mind is definitely not a brick and can predict his action with far better than random accuracy.