It takes a lot of courage to call the police on someone you love. I use to think that it would be devastating for my son to get a dui. It wasn't. It was an important step in his (and mine) self discovery.
I don't think he can find out who called it in. I don't think the 911 operator would ask for your name and if they do, you can always say that you'd like to keep this report anonymous. I'd be willing to bet they get calls like that all the time.