Try the following
the channel 4 Sex education site
programme 1
programme 2
programme 2b
This is worth going through as well
Big list of sexaul health, youth and sexaul health services
You also need to pre-empt playground nonsense myths your kids may pick out by getting in there first being open making sure he knows he can discuss anything with you or another male (dad, brother, family friend who can pull girls etc)
So he does not pick up nonsense like
Sperm dies in air
Pulling out can prevent pregnancy (This belief is responsible for most teen pregancies as schools dont teach about precum sperm leaked from the penis ALL THE TIME ITS ERECT... 1,000,000 per drop before ejeculation),
Any nonsense about safe times of the monh, cant get pregant first time, virgins cant have stds (many are blood transmitted ie fights, sport, first Aid or can enter the eyes)
etc
You also need to cover things like is normal for boys and girls to masterbate so he does not think he is a perverted freak likewise that males tend to get facinated with breats and thats simply part of human attraction likewise females end to prefer muscells of a guy.
(that way you dont end up with a kid who is a basket case needing years of councelling)
Much of Europe where they have a tiny teen pregancy rate compared to the uk... the worst in the UK so clearly most in the UK are wrong in saying you should wait and teach as little as possible) starts sex and relationship education aged around 4 and 5 and its part of the curriculum ill late teens so we are talking hundreds of hours of lessons.
It clearly works
Supporting statistical data 1
supporting statistical data 2