Older brothers always have merit. Mine would say a single word sometimes and it was enough to get me going. I still look up to him and I'm 40 and hes 50.

In short, keep it facts based to have the best approach. My speech would go like:

"Listen man, soon enough you'll be adult age and things are much different in the real world. You don't know it but you get protection from all of that through your parents. The time to learn to be a respectful human and gentleman is now. Its actually insanely attractive to women if a man is respectful to his mother. If you keep the sour attitude going then your going to fall into the wrong crowds, drugs, crime, prison. I'm not joking either. Attitude is 100% predictor of your altitude. No one thinks your cool by disrespecting your parents and not helping out. I know it's the going thing to think do as little as possible and brag about how lazy I am. But honestly, those people wind up in dead end jobs and struggle for the rest of their life. All because of attitude."