The Shulhan Aruch (Orah Haim 470:1) rules that firstborn males fast on the eve of Pesah and the source of this law is Masechet Sofrim (21:3). At face value this requirement seems puzzling since nothing calamitous happened to the firstborn Jews in Egypt, and on the contrary, they were quite fortunate. Rabbi Yosef Messas (Mayim Haim, § 179) discusses this contradiction and even cites the Talmud Yerushalmi (sv. “Arvei Pesahim”) which suggests that the firstborn do not fast. He explains that this disparity resulted in the widespread practice for people not to fast and to discharge their obligation to fast by attending a Siyum (the completion of a tractate of the Talmud). A Siyum has the status of a Seudat Mitzvah, which has the Halachic ability to exempt people from certain restrictions in certain circumstances, including fasting.
Rabbi Ben Zion Abba Shaul (Ohr LeZion, vol. III, Hilchot Ta’anit Bechorot) rules differently and says that since the Shulhan Aruch was clear about the obligation to fast, a firstborn must do so. If fasting were to have a deleterious effect on the person, such as arriving to the Seder famished and being affected adversely by the wine, only then would he permit one to attend a Siyum during the day and to eat afterwards. Furthermore, he explains that the reason a firstborn must fast is because of the principle that when one is a beneficiary of a miracle, one is subject to a concomitant loss of merits. When the firstborn in Egypt were spared by Hashem, they lost some of their merits and therefore fasting is a hedge against that loss. Other rabbis explain that, even though Hashem passed over the Jewish homes, the Angel of Death could have acted recklessly and still killed the Jewish firstborn. The miracle was that the Angel of Death acted as planned and spared the Jews, and this is commemorated by fasting.
Rabbi Ovadia Yosef (Yabia Omer, vol. IV, § 42) responds to a question regarding whether eating food that was used at a Siyum, even though one did not attend the Siyum, could exempt one from fasting. He responds to the contrary and says that in order to benefit from the exemption to fast, one must physically attend a Siyum.
Summary: A firstborn male may rely upon attending a Siyum in order to be exempted from fasting on the eve of Pesah.