What should a married man with children do, who has discovered that his paternal uncle adopted him when he was five years old, knowing that his official documents and those of his family are in his uncle's name, and he is currently outside his country for work?

The matter of Islam has settled on the prohibition of adoption and a person attributing himself to other than his father. Therefore, your friend's uncle must repent, and your friend must change his name in official documents if possible, otherwise in his dealings with people. He should treat his uncle and his family as a cousin, not as a son, in matters of Islamic law.