It's not "THE" talk. It's a series of age-appropriate conversations starting around age 4 when they want to know where they came from. That's when we started. But it's not too late for you, but you're getting close. Your son already has body changes he doesn't understand, and you need to get the jump on this.
They won't come to you if they think this all gets handled in one conversation, vs. being part of an ongoing dialogue!
I'd call the children's librarian of your library and say you want to get started but don't know how. Get a series of books for your kids, give their ages, take out a few books and then a few more. You can order through inter-library loan usually, and pick up/return at your own library. A book is a good starting point. Get one with a parents' guide that gives you suggested questions and discussion points, and limit your conversation to 10-15 minutes no matter what you leave unaddressed.
For ages 10 and up, the car is a really good place to discuss things, believe it or not. Not for every conversation of course, but for some. They can't get up and walk away, but they don't have to look at you either. Sometimes that's easier in awkward moments. Be alert to comments, movies, things you hear on TV that are conversation starters. Make sure it's not just a lecture, but a conversation about what your child knows, thinks, feels, and fears.
Once you find a few good books, leave them in your child's room for reading/exploration on their own. And start to be alert to things like desire for privacy, personal time for exploring their own bodies, and private time in the doctor's office during a check-up. Start leaving the room after the Q&A with the doctor, and let your children have part of that exam time with just the doctor.