113:6 Firstborn sons, whether first-born of the father or of the mother, fast on erev Pesach even if it falls on Friday. One who was born after a miscarriage also needs to fast. As long as the firstborn son is under the age of bar mitzvah, the father fasts in his place. One is allowed to eat at a meal that is a mitzvah if this is the accepted practice in one’s community.
113:7 A firstborn son who is fasting says “Aneinu” (“Answer us…”) in the afternoon service. If many firstborn sons are praying with the congregation, one of them should not lead the prayers because one does not say Aneinu in the repetition of Shemoneh Esrei during the month of Nisan. (If a firstborn does lead the service, he says Aneinu in the prayer of Shomeiah Tefillah, where an individual normally says it – Mishnah Brurah 470:2.)