YouTube just updated their app recently, there might be a complication between that and his device.
shadow wolf: it would be very helpful if you would give us your location, and the brand/model of device we are talking about. and the Carrier you use also.
If you can sign into youtube via a web browser, you just have an app issue on that device.
I would suggest you power it off, force it to do a bootup into the Service Menu, then select Clear Cache Partition.
Also, either remove the YouTube app, or at a minimum to to Play.store and update it with the new version.