Thank you all for your input. This is great discussion material.
The last thing I want to do is discourage my kid from going hard and working to score. In hindsight, with an inexperienced opponent, he should have probably just cut the guy and worked on his take downs until a pinning opportunity presented.
Now, with that said, I can see why that return could be called illegal. I'd still like to find some case examples to help me learn more. As someone else mentioned, that same return is normally not called a slam. If the defensive guy had landed on his head disproportionally from his body, or if he landed on his back, I would say easy call. To me, this one seemed like an overreaction turned into a judgement call and it didn't go his way.
Wrestler A has been developing nicely and hopefully he will learn from this. When I first saw this video, I told him not to change anything he is doing. After reading this discussion, I think that was the right decision overall. He didn't intend on causing injury and he will not in the future.
I do question and condemn any coach who will take an undeserved win at this age. I am not saying that this was an example of that by any means. I'm just saying......
Thanks again for the input.