Thank you for posting the issue on the Microsoft Community Forums.
From the issue description I understand that you are facing an issue playing YouTube HTML5 videos.
To assist you better I would appreciate if you could answer the following questions:
1) Have you made any changes to the hardware or software on the computer prior to the issue?
2) Do you get any error message / code?
3) What is the make and model of the computer?
4) Are you able to play the same YouTube HTML5 videos using the Internet Explorer browser?
5) Which operating system is installed on your son’s computer?
Refer to the link and follow steps and check,
What to do if you have problems with an app.
I would suggest you to download and run the app troubleshooter and check.
Refer to the link,
Also I suggest you to contact the YouTube Help Center.
This information should help. If you need further assistance regarding Windows, please post. We will be happy to help you.
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