It's a decision only you can make. If you bothers you, you should let him know. People do things they think are cute and funny but others find it annoying and boner-wilting.
Maybe nobody's told him that his 16 year-old's nickname for his dick is not appropriate now that he's older, and that when he says it, you're put off and it kills the mood. If he's more mature than his cock-vocabulary suggests, maybe he'll take the feedback and change his ways.
If he doesn't, then it's up to you to determine if it's something you can live with. Yes, somethings are trivial and shouldn't matter, but irritation doesn't always lead to a pearl... sometimes it leads to a wound that never heals. But only you can make that choice.
Likewise, it's up to him on changing. Maybe you're worth him giving up a stupid nickname, or maybe he loves saying that stupid thing more than he cares about how it turns your stomach. In which case, he'll have made a choice as well.