Taame Haminhagim 903 starts off by saying he's going to explain why it's called a shalom zachar and proceeds to cite Tosafos (Bava Kama 80:1 s.v. "L've") as saying that the reason the g'mara there calls a shalom zachar a "salvation of the son" is that he was saved from the womb (citing "וְהִמְלִיטָה זָכָר", from the haftara of Shabas rosh chodesh, related to words meaning "escape"), but then doesn't, as far as I can tell, complete the thought about why it's then called shalom zachar, instead giving what is, as far as I can tell, a completely separate reason: that it's called shalom zachar because it takes place on Shabas which is called shalom. It seems very odd to me, so perhaps I'm reading it wrong, but that's what I'm seeing.