If Tisha Ba'av fell out on Shabbos, and therefore it was pushed off to Sunday. Is a Bar Mitzvah Boy obligated to fast if he turned 13 on Shabbos? In other words: Does the fact that Tisha Ba'av fell out on Shabbos and now moved to Sunday constitute the Boy obligated in the fast or not? Please source.
The Kovetz Halachos writes that he should fast if possible. He brings the opnions on page 21: footnote 17. The Avnei Nezer 426 was muspak about this ,and explains whether it's considered a tashlmin or not . He seems to come out that it is and a katan would be patur. The Maharsham 3:363 also seems to be maikel, however there still may be an Inyun by chinuch when it comes to tisha bav. The Divrei Malkeil 5:130 argues on the Maharsham and holds that a katan is chayav to fast since its not considered a tashlumin. Rav Asher Weiss also held that it's not considered a tashlumin, but rather it was instituted by Chazal to be pushed off ,so one would be chayav.