His prayer is valid; because the condition is to cover the private parts from the sides correctly, it does not harm if someone looks at his private parts from under his garment; because the effort to prevent this is a hardship. See: Maraqi al-Falah p. 210-211, Al-Wiqayah and its explanation by Sadr al-Shari'ah 1: 143, Radd al-Muhtar 1: 408, Bada'i al-Sana'i 1: 117, and Al-Hadiyyah al-'Alaiyyah p. 72.