Parents gotta parent. A kid's battlestation and online acquaintances are no replacement for parental attention. If a kid is taught well by their parents they should be mentally equipped to be wary of sketchy folks and their wacky ideas, regardless if in the outside world or online.
the fact that this 15y/o kid's father was so utterly flabbergasted that his son had an interest in nazism is a hint to me that they didn't have much meaningful talk with one another. Casual talk about ongoing events and news for example could easily disclose hints that his kid harbours rightwing views.
Being overwhelmed to the point of crying about finding some nazi propaganda ain't a good parental reaction in my eyes either. He should've rather talked calmly with his son about these papers and their content, ask what he thought about them, if he believed in what they said and why, and explain his view against them, how racism and nazism are flawed and how deranged the people are who propagate these destructive ideas.
Online moderation on a large scale is bound to be resource-intensive and flawed; people will find ways to circumvent it no matter what, for example by coming up with new codewords and dog whistles to avoid censoring, like for example tipping $14.88. Or they move their communication away from the game's built-in chat to third party chats, e.g. discord groups, where moderation is even less likely to affect them.
Last but not least we gotta stop to enshrine rightwing extremism on the pedestal of forbidden thought. To forbid something because its so evil and dangerous, to the point that even reading or talking about it is shunned, just empowers it through an aura of mystique and shock value – and little is more effective in drawing edgy teens' attention to a subject than declaring it dangerous and off-limits.
Just as the Satanic Panic of the 80's lead to some teens getting interested in edgy, socially forbidden occultism, nowadays some teens get hooked on nazism just for the same reason of being dark, edgy, and forbidden.
Nazism shouldn't be treated as a serious threat that has to be socially prohibited, but rather thrown together with all other failed, wacky, discredited and largely rejected ideas, which we regard nowadays merely as matter for laugh and mockery, not serious fright.