Just one of those things you are going to have to leave for him to work out really, you won�t be able to interfere or control that part of his life! He might have started when he was 11. It�s a bit like girls and periods, you never know when it is going to start and everyone is different.
A man and his willy is a very intimate relationship � one that a woman could never really understand! From a very young age a man grows up with the dangly bits hanging there, stuck on the outside, there is no hiding them, they bulge out, we have to hold them to wee, readjust them when they are uncomfortable, and conceal them when they are becoming obviously excited! It can all be very embarrassing, without chatting it over with mum.
As you have said yourself, its personal. He can learn a lot more from his mates, from books and magazines and the girls behind the bike sheds than he would ever want to discuss with you. So, just let him get on with it.