Do teenaged guys just randomly get s during the day?

Yes, this happens to every man. (Except for those without one) Just don't pay attention to it, that will help it die down a little bit. Does he wear loose jeans? Because I think that amplifies it a little bit. I remember the other day in LA I had a rock hard *****, and right when it got completely solid, we had to get our books. It was pretty funny, because there were a lot of girls there, and it was really showing. I think the only difference between me and your guy friend is that I get turned on, and think it's funny when that happens, but he just gets embarred about it. If he brings it up, just tell him not to worry about it.