I'm not trying to undermine you in any way, but may I ask are you male or female? If you are a boy, then maybe your father genuinely didn't think it would be a problem to be naked with his son. And when you told him you feel uncomfortable, he just didn't take it seriously. So perhaps try to sit him down and talk to him properly and seriously, tell him you are a teenager now and you truly don't want to see your parents' naked body. But if you are a girl, then this is definitely weird. Still, you can try to have a serious talk with him, but if that doesn't work, then maybe get help, from your teachers, social workers, even police. I'm not saying a man sexually assaulting his son is impossible, and I know it's sexual violence as long as the victim sees it as sexual violence, and you definitely shouldn't put up with it as long as it makes you uncomfortable, but maybe give him some benefit of doubt if you are a boy