What is the ruling regarding someone who had intercourse with his wife after suhoor and Fajr prayer on the last night of Ramadan, thinking that abstaining from nullifiers of the fast only begins after the white thread becomes distinct from the black thread of the day, and what must he do?

Whoever has intercourse after the true dawn in the daytime of Ramadan, while ignorant of the time when fasting begins, is not liable for expiation but must make up that day. The excusable ignorant person is one who has recently embraced Islam or one who grew up in a remote desert area far from scholars, such that the matter is obscure to him. As for one who has mingled with Muslims, he is negligent and his ignorance is not excusable; he must make up the day.