Am I obligated to help my fiancé overcome his addiction to pornography websites, and would this be considered a sincere endeavor for the sake of Allah? And what are the possible practical ways to help?

If the suitor watches pornographic films, he is not pleasing in terms of religion and character, and a believing woman should not marry him unless he repents sincerely with its conditions, and his conduct becomes upright as witnessed by trustworthy individuals. If he does not repent, it is preferable to annul the engagement. The fiancée's interaction with him should be very limited because he is a stranger to her, and she can pray for his guidance.