It depends on you relationship. If he is a good boy that wants to please, you can have a heart to heart conversation. That should work, especially if it was the first time. For him it is fun and an experiment, while for you it is horrible, dangerous, and wrong. Give him the big picture.
There are other ways to react. If you don’t feel like he is in your hands, you might have to leave it. I hope he has a Rebbe that he and you trust. Speak to him.