I remember once I have seen 「なの」 was not placed at the end of a sentence (meaning it was not a question) ever since then I wondered what it meant but didn't have any clear examples. Today I saw this sentence in my textbook:
男の子なのは知ってる
The textbook translates this sentence as "It is a boy, I know". The topic of the chapter is not about the 「なの」 part, so no explanation is given about it.
What does 「なの」 mean here?
Edit 1: Got the comma right in the translation. Sorry for misunderstanding!
You can break なの down into the formal noun の and the adnominal copula な (i.e., a form of だ which shows up before nouns).
Your sentence: Basically, the sentence （彼が）男の子だ is embedded into は知ってる by turning it into a noun using の.
（彼が）男の子だ
⇒［（彼が）男の子な］のは知ってる
Alternatives: You could also write it as
男の子であるのは知ってる
"I know that he's a boy."
which is equivalent in meaning but slightly more formal/literary/whatever because it uses である in place of な to represent the copula. Another option is
男の子（だ）ということは知ってる。
"I'm aware of the fact that he's a boy."
which gets across the same point in a slightly more lengthy way.
If it was simply 男の子は知ってる that would either be read as "The boy knows" or "I know the boy", because there is no copula.
