M.G.
You don't say why you are in this position - of practically raising your nephew.
Personally, I don't find being disrespectful, having an attitude, talking like he owns the place, talking back (just being disrespectful in general) is 'acting like he's grown' or an adult.
He sounds like he is craving attention from his parents - and I am guessing there is some kind of turmoil going on in his life.
Kids act out when there is not enough consistency and structure, attention and love in their lives. They can if there is divorce, upset, etc. They can for other reasons, also, and sometimes it is just a phase, or testing limits.
If you are babysitting - then you can just enlist his parents' backup - withhold treats, TV, rewards (whatever reward system there is - playing outside, going to park, reading stories, etc.) until he behaves. Reward good behavior, ignore the bad. That kind of thing.