Fasting on the Yahrzeit of a Grandfather

If one's paternal grandfather passed away after one's father, is it necessary to fast on the grandfather's yahrzeit?

Even though the grandson should recite Kaddish, he does not need to fast. It is advisable to increase Torah study and good deeds in memory of the soul.

Regarding Kaddish: Pithei Teshuvah, Yoreh De'ah, Siman 376, Se'if Katan 7 "Agavei Teshuvah" of Rabbi Maharit, z"l, Se'if Katan 11, in the name of (Teshuvot Rama) that if he has no son, it is appropriate to give Kaddish to his grandson, but other mourners will say two Kaddishes and he one. And see (in Shu"t Teshuvah MeAhavah, Part 1, Siman 178) where it is written that in a place where there is no mourner for a father or mother, and there is a grandson who says Kaddish for his grandfather, and there is someone who says for one of his relatives or is hired to say Kaddish for someone, the grandson takes two parts and the other one part. And it is further written that there are many details to consider if one says for his grandfather and another for his father-in-law, or one for his elder brother and another for his grandfather or father-in-law, and so on, but I do not wish to elaborate on what has no root in the Talmud. And so wrote the Mishnah Berurah in Kuntres Kadishin. However, there is no obligation to fast as we do not find in the sources that one must fast, but it is still advisable to increase Torah study and mitzvot in memory of the soul (and perhaps it depends on the reasons brought by the sources, whether the reason is due to a bad omen or for the needs of the deceased, as the grandson's fate does not worsen, and perhaps it does not elevate the father's soul as much as the son, therefore one should increase Torah study and mitzvot in memory of the soul).