Younger boy cousin masturbates in front of me while watching TV...

Hello. I have a bit of a problem with my younger cousin. He’s a 9 year old boy, and as we all know, 9 year old boys are….how do I say it? A little gross. Well, here’s my problem. Every time he comes over, he sits in the living room and watches TV. While he is watching TV, he starts to masturbate. Like rubbing on the couch and his hand down his pants, IN FRONT OF ME. I think that he is old enough to understand to not do that in front of others. And I really don’t know what to do when this begins, it’s just so fucking awkward. I would really like to know if there is anything I can say or do to let him know him know it’s not okay (I don’t want to traumatize him or scare him or anything like that). I really don’t think I can go to his mom, because I’d just feel weird about it. So Askreddit, please tell me is there anything I could/should say to him? I would really like this to stop.

Edit: I am a 20 year old female, if that helps.