What is the most appropriate way to advise an uncle who does not pray, given that he has a strong personality and is difficult to convince?

The call to prayer should be made with gentleness and kindness, reminding the one who abandons it of Allah and the Day of Judgment. One should seek assistance in this through impactful tapes and lectures, associating with righteous people, and not despairing of their guidance, for hearts are in the hand of Allah; He turns them as He wills.