Judge the risk of him finding out from someone else. I think only you can decide that, based on your situation and peers.

When he finds out, it should be from you. And you don’t have to go into specifics when you tell him.

Picture it from his perspective. If one of his friends (who found out from their parent or something) tells your son, do you want him to be surprised you were keeping a secret based on how some teenage boy describes it? Or would you want him to already know the basics from you?