First, if it's just a stupid joke among peers, you can have a conversation with those involved about appropriate language and respect. Tell them it's not okay to use such vulgar terms in relation to your son.
Writing or seeking such fanfiction is inappropriate and goes against the general positive and family - friendly nature of the Naruto franchise. Fanfiction should focus on positive aspects like character development, new storylines within the acceptable moral framework.
Well, without more context, it's hard to say exactly. It could be that there is some sort of object or situation that has been misnamed or misrepresented as a 'huge cock'. Maybe your son has a large - sized item that someone is joking about in a very improper way. But we should always be cautious of using such vulgar language when referring to children.
Well, if not in a vulgar sense, 'huge cock' could potentially be a misnomer or a very strange way of referring to something large. Maybe it's a big rooster on the family farm that the stepmom has some sort of story related to. But this is a very odd and uncommonly used way to phrase it if not being inappropriate.
I'm not sure specifically which 'huge cock story' you are referring to. There could be many stories that might use this phrase in a non - vulgar way, like a story about a large rooster perhaps.