I did a search on H.com and found this:
I didn't credit all the quotes but they are all found on two threads. One...Two
I once read (questions and answers in The Jewish Press-don't laugh) that one of the reasons for a Shalom Zachor is to fufill the mitzva of bikur cholim with regaurds to the mother.
I looked into the issue, and apparently the main reason for Shalom Zachar is to give the baby well wishes before the bris, which is physically demanding
I thought the reason for a Shalom Zachar is to celebrate the baby's first Shabbos, and the reason why it's for a boy is because he can't have his bris until he lives through a Shabbos and absorbs the special kedusha from it.
reason for a shalom zachar is because when the baby is in its mother's womb, it is taught the entire Torah. At birth, a malach(angel) touches the baby at the spot right above the upper lip and below the nose(that indentation called the filtrum)and the baby forgets all the Torah that it was taught (the reason is once you've learned something and then forget it, there's an incentive to try and relearn it again. This loss of Torah knowledge is a form a mourning so people get together on Friday night as a form of "nichum aveilim". The reason Friday night is chosen is simply because everyone is available(i.e. no work) to come by. Also, Friday night is chosen because the term is "Shalom Zachar" -Zachar means boy but has the same root as Zachor( i.e. Zayin, Chaf , Reish) as in "Zachor Et Yom HaShabbat L'Kadsho. To reiterate the mourning theme, that's why chick peas(Arbis) are served at the Shalom Zachar(they're round hence the circle of life theme)
according to the Noda BiYehuda it was established to let people know about an upcoming bris without letting the Romans know?
Does anyone have a source for any of this? Does anyone know of other reasons not mentioned here? I need to prepare something to say.