You need to enforce the rules. He should respect you and your house. If he can't do that much then he needs to leave. It isn't fair to the rest of your family to have to put up with his behavior of self destruction.

I think he thinks that you will never threaten and kick him out of the house. However, he is also about to get married and needs to learn some responsibility. If you need to, have him start paying rent to you and make a written agreement of the things he is and isn't permitted to do in the house. On the agreement you can include a fee for every time you find beer/bong/mj in his room. It might discourage his behavior a little bit.

It sounds like he is arrogant and doesn't know anything about the real world.