I say, and with Allah's success: He is not obligated to perform I'tikaf at night due to the lack of common practice. He must enter the mosque before dawn and remain there until sunset; because he committed to I'tikaf for the entire day, and the day is defined as the time from dawn to sunset; as evidenced by fasting. He is also required to fast on this day; because fasting is obligatory in the obligatory I'tikaf that is caused by the vow. Refer to: Al-Tahbiyyin 1: 353, and Al-Mabsut 3: 122, and Allah knows best.