If someone had an abortion prior to giving birth to her first living child, a son, is that son not a Bechor which would require a redemption? My assumption is that he's not since the Bechor is supposed to be the opener of the womb.
Whether to concider a stillborn as the firstborn or not would depend on the stage of developement of the fetus and other variables, See Shulchan Aruch, Yoreh De'ah 305:22 & 23, with various commentaries and their opinions. But if you are discussing an early term abortion, before the fetus has human form than all would agree that the next child would be the bechor.
You are correct.
המבכרת המקשה לילד, מחתך אבר אבר ומשליך לכלבים.יצא רבו, הרי זה יקבר , ונפטרה מן הבכורה.
An animal that has a difficult time birthing its first offspring, the fetus can be cut into pieces and thrown to the dogs (Bartenura -- it is not holy). If most of it came out, it should be buried (Bartenura -- it becomes holy as a firstborn when most of it comes out). The mother is exempt from בכורה (first born).
The last Bartenura on that mishna explains:
שהבא אחריו אינו בכור. בין שיצא ראשון אבר אבר ובין שיצא רוב כאחד, דהא שני לאו פטר רחם הוא.
The one that comes after it is not a בכור (firstborn); whether it [the firstborn] came out most at once (and became holy) or whether it came out in pieces (and was never holy), the next one is not the פטר רחם (one that opened the womb).
( all translations are mine )