For the love of God, please help me! What do I do about this boy in my English class?

So this kid- let's call him Adam- doesn't have very many friends. He's done bad things before, but I decided to give him another chance and sat next to him in English 11 because I know what it's like to have no friends. His dad is my orchestra conductor, and he's probably one of my favorite teachers of all time. Adam, however, is not. He talks during class and copies off of my papers, both of which I have talked to him about at least once in the past ten days of school. But he also masturbates during class sometimes- I heard from some of my friends that he did it during play practice, but it sounded so bizarre that I figured I would have to see it to believe it. Well, now I have seen it and definitely believe it! Doing it privately is perfectly acceptable, as he's 16 and most people our age do things like that, but not in public, and certainly not in class! My friend told me to tell his dad, which I definitely don't want to do because, really, even if it was one of your favorite students who said it, you wouldn't believe something like that about your kid, right? Should I talk to Adam about it and risk him telling his dad? (He talks about me to his dad; I know because he asked if I was keeping his son in line the other day and said that he told him that I sit next to him) Or should I just tell the teacher? If so, how? Please help me- this is a traumatic thing to witness, without question, and I'm not sure how to get him to stop.