Learn English – Word for someone who is driven by morality

Unflappable describes someone who remains composed in the face of adversity or scandal, "impossible to fluster."

Stoic has the same meaning, but often implies indifference to pleasure as well as adversity.

He could well be a Moral Absolutist:

Moral absolutism is an ethical view that particular actions are intrinsically right or wrong. Stealing, for instance, might be considered to be always immoral, even if done for the well-being of others (e.g., stealing food to feed a starving family), and even if it does in the end promote such a good. Moral absolutism stands in contrast to other categories of normative ethical theories such as consequentialism, which holds that the morality (in the wide sense) of an act depends on the consequences or the context of the act. (-- Wikipedia)