That's not really a moral code situation though, that's an action. I would absolutely make it clear that action is not acceptable under any circumstances and would enforce that. But that's different to him getting his sense of morality or moral code from me. Because that's much deeper, and doesn't just relate to one action. It's about why such actions are wrong - in this case, having respect and care for other people is the underlying moral belief that I have which underpins why you shouldn't bully anyone. And if I was asked why on that, it would go to my underlying moral belief that my actions should always try to make the world a better place, or at the very least, not a worse place.
It's a philosophical question, really, just drilling down to the axiom that underpins your morality. Do you actually have a consistent thread that links together your moral beliefs, or is it just because your parents, God, teacher, faviourite YouTuber told you so. I want my son to have a consistent thread, and he's only going to have that consistency if he finds it for himself. For example, if he continues to be religious, then his moral code has to be different from mine - as my moral code is build on the belief that there is no God which means there is nothing beyond death. So for me, the alleged actions of Mason Greenwood are wrong because of the harm he is causing to another individual which makes the world a worse place. For a religious person, the actions of Mason Greenwood are wrong because God says it's wrong and all morality comes from God. As long as my son grows up to think those actions are wrong, I don't care what his moral code is which gets him there - I just want him to have a clear reason as to why they are wrong.
I suppose the only real disagreement we have on this, is that by getting your child to see their parent as a role model, you can stop them seeing a footballer or YouTuber as a role model. I don't think it is and I think you're underestimating the influence that a footballer, YouTuber or any kind of celebrity can have. It's just natural for someone to see a successful person and want to be like them in order to achieve the same success. This is especailly true for young people. While they will look to their parents, their parents will 99% of the time not have achieved success in an area that the child wants to be successful. So they are going to treat them as role models, whether you like it or not. If Mason Greenwood was to sign for Boro and fire us to promotion this season, I would absolutely expect that my son would look to him and think "I want to be like him." For me, if his moral framework is just based on "because my dad does this", that could easily be replaced with "because Mason Greenwood does this", whilst if the moral framework is "because I shouldn't ever harm anyone" then that can't be replaced in the same way.
We're all suceptable to being influenced by celebrities anyway. There's a reason why political movements and parties want to spend so much money on getting their message out there and try to make their spokespeople "influencers".