Whatever it is, Youtube censors that stuff for underaged kids. Your child is thirteen, which means that he should be able to make a Gmail account if he hasn’t already. Enter his real year of birth when filling out information, and it will block anything inappropriate until he is of age
Why don't you install any parental control app on his phone/lap top? In my family we use an app called mspy to supervise our son`s Internet activity, view the list of all visited websites and block the unwanted ones if needed. We also can block app we find inappropriate. First our boy tried to delete it but failed and stopped his attempts.