Thank you for posting your question in the Xfinity Forum. We apologize for the inconveince that you are experiencing with YouTube on X1.
I can tell you that what you have done to remove the app and access to it is absolutely correct. When testing on our end we are not able to replicate this issue. Is there any way that your son may have accessed the other YouTube App on X1 (YouTube Kids)? Have you disabled this app as well?
If you have disabled both YouTube apps and are still experiencing this issue, please send me a private message with your full name, account number, address, and phone number so we can investigate futher.
To send a private message, click on my name and navigate to my profile. Once there, click the "Send a Message" button.