What does 'cuz' mean?

Cuz means cousin. He’s not using it correctly. I.e. - “What up, cuz?” Is an example of how it should be used where cuz is the thing being addressed. Generally, this manner of speech is associated with Southern Californian African American Vernacular English. More specifically specific with members of the Crips gang. There might be other groups that use it like this but that’s who I’m aware of. I wouldn’t call it inappropriate at his age but unless you live in a rough neighborhood or he has classmates that live in a rough neighborhood; I would guess he’s picked it up from the internet so it would be best to monitor and limit his screen time. I don’t know about in other parts of the country but in Southern Los Angeles for example using gang speak in the wrong neighborhood could get you mugged or worse. Not at 6 but by middle school kids from those neighborhoods could start being affiliated.