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.
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.
I'm not sure if it's appropriate to specifically focus on the size of a pet rooster in stories. However, if your son has a large pet rooster, there could be stories about how it struts around the yard, wakes everyone up in the morning with its crowing, and how your son takes care of it, like feeding it and making sure it has a clean coop.
There are also some illustrated children's books where a big rooster is the main character. The story might revolve around the rooster's adventures because of its size. It could be that it has trouble fitting into the chicken coop or has to find special ways to interact with the smaller animals due to its larger stature. This can teach children about differences in size and how to adapt.
Your son could have an adventure where he takes his rooster on a hike. The rooster might explore the new terrain, peck at interesting bugs or plants it finds along the way. Another adventure could be when they participate in a local fair or pet show. The big rooster would surely attract a lot of attention.
Well, if it's not about something inappropriate, perhaps it could be a story where your date showed extreme confidence, like a 'huge' personality trait that's being described in a rather bold way. For example, they might have been really assertive or had a large presence during the date.