Serious ques. about boys and their privates!!!?

I am a nanny and watch 3 childern, so this is a big shock to me when this occurs. He is almost 8 always has his hands in his boxers and rubbing. Whether he is out with me at the zoo or talking to a friend, at the computer playing a game, I mean a lot. This week I walked into their living room and he has his clothes pulled to his ankles and the warm place on the couch from their skylight. Since I have worked for them for some years know we all are aware of this occurence. He is not supposed to do this unless in his room with the door shut ( so the other kids don't literally sit next to him while he does what he does) . His parents have always blown this off. I am wondering if this much "rubbing" is normal. Just want to get some peace of mind of whether to agin bring this up to his parents that don't seem to mind this behavior.