Phrase Meaning – Can ‘Oh Boy’ Be Said to a Girl?

I've actually profitably made heavy use of the phrase "friendly reminder" in email and text. As in:

Friendly reminder: your appointment with me is at 4pm this afternoon.

or

Just a friendly reminder, your appointment with me is at 4pm this afternoon.

The other thing you can do, which I also do, is to skip the preamble about how they might have forgotten, and simply reiterate the information they might have forgotten -- or otherwise slip it into the conversation:

I look forward to seeing you at our 4pm appointment later today.

A related trick is to include a reminder, clarification or request about something else entirely which happens to include the stealth reminder for what you're really concerned about:

I wanted to let you know, the ceiling projector is out of order in the room in which we're meeting today at 4pm. I thought I should mention in case you were planning on using it.

Hey, would you happen to have some spare dry erase markers you could bring to our 4pm meeting today? The conference room was out when I was there earlier.

Another trick, one which I don't use personally, but have seen others use, is to pretend one might have forgotten, oneself, and ask the party you suspect of forgetting for confirmation:

I'm sorry to bother you, but I think I may have written our appointment time down wrong. When did we agree to meet?

Yet another approach I use with my sometimes flaky boss, regarding our 1-on-1 meetings, is to ask, as if no commitment had been made, whether something is true (typically via SMS which is the only way to get him when he's in other meetings):

Are we meeting at 4pm today?

The nice thing about this approach is it lets him save face if he forgot and made other plans: he can just write back "No, not this week" or "No, I have to debargle the overthrusters" or "No, let's meet next week". He doesn't have to admit he forgot. Also, it doesn't make me look like I'm nagging him, the way sending reminders might.

There are two cases for (my) son:

1. Church context:
   One reason for celibacy in the catholic church is, that the priest should not be "distracted" by his own family, but consider his congregation as his family with him in a paternal role. Hence addressing him as "father XYZ". So it's perfectly acceptable to call his parishioners "my son" or "my daughter". This father/child addressing works in many different languages because it expresses an understanding of their relationship and tasks. The "my" is fine here.
2. Seniority, often protective:
   Usually used without the "my", the address "son" expresses the - temporal, usually just for the moment - assumption of a father-like role (or mentor) by the speaker to the addressee. Think of "it takes a village to raise a child" (from a different origin, but same thought). It always carries an undertone of seniority, often reflected in the age difference. Normally, the speaker would use "son" in a well-meaning way, even when admonishing. This seems only to work for boys, though, I have never heard the phrase "daughter" in a similar context. "Kid" might perhaps work for girls.

EDIT:
just for the sake of completeness, as requested ^_^:

3. Expressing the social or biological relationship:
   Father or mother talking to their son. But I'm quite sure that's not what OP was asking...