Did not accept the fast at Mincha before the Yahrzeit and began fasting, should he say 'Aneinu'?

Someone who was not accustomed to fasting on the day of his mother's Yahrzeit and began fasting, and at noon he learned that it is not a fast since he did not accept it upon himself, should he say 'Aneinu' at Mincha?

For those who follow the opinion of the Shulchan Aruch, he does not say 'Aneinu', and for those who follow the opinion of the Rema, he says 'Aneinu'.

It is stated in the Gemara Taanit, page 12a: "Shmuel said: Any fast that was not accepted upon oneself beforehand is not considered a fast. And if he fasts, what then? Raba bar Sheila said: It is like a bellows filled with air. When does he accept it? Rav said: At Mincha, and Shmuel said: In the Mincha prayer." And the law follows Shmuel, and the Shulchan Aruch, Orach Chaim, Siman 562, Seif 5, writes: "Any fast that was not accepted upon oneself beforehand is not considered a fast." And the Rema writes to pray 'Aneinu', but if he simply fasted, it does not fulfill his vow. However, there is an opinion that one prays 'Aneinu', and this seems to me to be the correct practice for an individual fast." The Mishnah Berurah explains that there is a dispute in the Gemara whether it is not considered a fast in terms of not fulfilling his obligation, or even in terms of 'Aneinu'.