I've heard that some men fast on erev Pesach for their firstborn son until that son old enough to fast for himself. Is that halakha? Where does this come from?
Rama in Orach Chaim 470:2 says that a father fasts for a son who is a Bechor
Many authorities, including the Rema (OC 470:2), note the custom that the father of a firstborn should fast on his child's behalf until the child reaches halakhic adulthood. The Rema rules that if the father is a firstborn himself, the mother should fast on behalf of the child. The Mateh Moshe and Maharil dispute this and rule in such a scenario that the mother need not fast. The Magen Avraham rules that it is appropriate to follow the lenient opinion if fasting causes the mother excessive discomfort or if she is pregnant or nursing, but he adds that a mother who begins following the former opinion must maintain that custom and fast in subsequent years.
The Sh'vut Ya'akov (1:17) rules that the above-cited custom of the father fasting for the child goes into effect as soon as the child is born, except where the child is born after chatzot ha'laila (halakhic midnight, which generally corresponds to solar midnight) on the 14th of Nisan of that year. (Since the child had not yet been born by the equivalent time that the Plague of the Firstborn had occurred in Egypt, the father need not fast for his child until the following year) The Korban N'tan'el (Pesachim 10:19:80) disagrees. He writes that the custom only goes into effect from the time the child is 30 days old. This relates, again, to the command to redeem the firstborn, which does not go into effect until the child is 30 days old.