Is it obligatory to repeat the entire rak'ah if one makes a mistake in a surah, or is it sufficient to repeat only the surah? And what advice do you offer to a young man who repented only four days ago?

If a worshiper makes a mistake while reciting Surat Al-Fatihah, he must correct the error and complete the recitation. If he does not realize the mistake until after bowing (Ruku'), he must stand up and repeat Al-Fatihah. If he remembers after he has finished the entire bowing (Rakah), that Rakah is nullified, and he must perform another one. As for mistakes made in recitations other than Al-Fatihah, there are no consequences, as reciting a Surah after Al-Fatihah is recommended, not obligatory.