I heard that this year, only first born males need to do a siyum masechet due to the Ta’anit B’chorim fast being earlier (moved up from Shabbat to Thurs.), whereas usually the fathers with first born’s also need to. Is this correct for this year and for both Sefaradim and Ashkenazim and Chabad?
The special leniency you mentioned is found in R. Ovadia Yosef (Yechaveh Da'at i, 91) and is based on the fact that the Shulchan Aruch even brings the opinion that when the fast is moved up, everyone need not fast at all. Ashkenazim clearly don't hold that way, but the fact that an Ashkenazic posek, R. Menachem Shteinhart (Divrei Menachem, 8) also is lenient when the fast is moved up to Thurs., in the case of a new father who has never fasted before (so he doesn't have a set custom yet) and he has to travel & can't hear a siyum. He writes that being that it's just a custom, one who is lenient has upon whom to rely in this case, but it seems that he infers that in a regular case without those special factors, if you can easily find a siyum, why not go? But if it's difficult, especially this year with Corona, then this would also be considered an extenuating circumstance, and you can surely be lenient in place of your son, even if you don't hear a siyum, or in his words: "the lenient one doesn't lose anything". [Remember also that R. Yosef Eliyahu Henkin zt"l, the posek of America before the arrival of R. Moshe Feinstein, posits that in any case, this custom to fast isn't that obligatory even for the firstborns themselves, and that's why we are regularly lenient to hear a siyum to exempt, and when need be, he even allowed to redeem the fast through giving money to tzedakah]. This would all apparently apply to Chabad, as well.