B.C.
Now is a good time to start.
It's never 'the talk' as in you-go-over-it-once-and-never-talk-about-it-again.
It's a series of conversations over the years.
Most start with what changes to expect when puberty comes on.
The first change? body oder and the wearing of deodorant - and that one will hit him in 3rd, 4th or 5th grade - certainly before he's out of elementary school.
There are many books that can help you.
Google 'puberty books for boys'.