he did not take his helmet off in the end zone... he was on the sideline
http://fansided.com/2016/12/17/n-c-central-loses-celebrating-celebration-bowl-video/
The OP article said he was in the end zone. Watching the video I concur with you, however - it looks like the kid was out of the end zone when he caught the ball as well.
The video shows he went out of the back of the end zone and removed his helmet. He was not on his own sideline.