What is the appropriate way to advise an uncle who does not pray, taking into account his strong personality and the difficulty of convincing him?

It is obligatory upon a Muslim to offer sincere advice, for guidance is in the hand of Allah alone. The one offering advice should choose the best methods and means for it. One should not despair of giving advice even if the advised person does not accept it; rather, it must be repeated and its means diversified, such as direct advice, or gifting a book or an audio recording, or seeking assistance from those whom the advised person respects.