I need help with my 18 yr old son.

He's 18. If he's living in your house stop enabling him. He either lives by your rules... or he gets out.

Yes, absolutely, if he doesn't follow your rules, kick him out and he can make his own way.

But you do need to stop thinking of him as a child who must listen to you as the parent. There is no authority. He is 18. And he will do what he chooses to. And you can't stop him, control him, or change him in anyway.

What you CAN do, is kick him out if he refuses to give up the drugs (what is cush by the way?)

Kush is pot.

And I agree. Tell him your way or the highway

What do you mean, he does not take authority well ? Does he threaten ? Does he stay out,
What type of help or work does he do around the house