Yes, mine knew at (and before that age). It will talked about at school, our is a very 'nice' school but we have a lot of doctor's children, who tend to speak very frankly about everything, and in DC's year we had 1 child who was unfortunately left to watch 18 videos, so brought a less scientific and far more worrying version to school.
With DC aged around 3-4 we started with explaining anatomy and in the broadest terms 'how babies were made'. They asked very early "exactly" how the sperm got to the egg. We don't sit them down for conversations, just take questions as they come, though we do occasionally introduce a new book (e.g. about periods, as DD9 could start any time) and then as they've no embarrassment, they continue to ask questions.
DD9 is aware that penis goes in vagina to make babies (she thinks this sounds horrible), when this is appropriate, healthy relationships, when to tell a trusted adult, periods, contraception (she asked), puberty etc, different types of family and relationship etc. Given that these days girls can start their periods as young as 9, the vast majority will be aware, even if your son is not.
For books, there are a few Usbourne one, starting with basic lift the flaps about the body, with a page on babies etc, to 'what is happening to me?' type books around puberty, and also (probably a bit young for your son), "What Makes a Baby" which is a very simple, inclusive book that covers many, perhaps all, permutations of couples.