I don't understand the problem. What's the problem with letting him know what's going to happen to his body as he gets older?

We discussed body changes and the facts of life with our boys when they were about 7. One of them was fascinated, the other was not interested. I think the more this is demystified, the better. It has not harmed my boys, nor did it change their relationship with girls, if that's what you're worried about.