The referee made the call without hesitation or doubt and was in superb position to observe the action. Further, if this is an 8th grader doing the slam, then the referee did an even better job.
As to better techniques, there are many things to improve upon;
-the riding position (video was very short)
-use of a foot stomp or trip to return him to the mat
-learning to avoid a dangerous move
-learning to be concerned about your fellow competitor (video was short, but difficult to see any apparent concern)
I don't think a criticism as a sore loser is supported, but the posting and video sure make a case for an opportunity to improve sportsmanship.
I'd work on sportsmanship, especially with an 8th grader, and the rest will follow.