Most Helpful Guy
It sounds like he's an alcoholic. He's addicted and there is nothing you can do for him. He'll change if / when he's ready. Odds are against him changing though.
I realize you've been with him a long time and have kids. That has to be taken into consideration. But I'm still tempted to say you should leave him. I went through it with my ex - don't waste years of your life waiting for something that will probably never happen. Get out and find someone else while you are still young. I wish I had done that.
You can give him an ultimatum that he either quits or you and the kids are gone. You can't make empty threats though. You have to be prepared to go through with it, even to the point of making solid plans. Are you financially able to take care of you and the kids if you leave?
He needs to know that it's very serious. He's old enough that it's not just typical youthful partying. He has a family to look out for and he's dropping the ball. The alcohol and drugs are stronger than his will power. Don't let his drinking and drugs pull you and the children down, you deserve better than that. Give your kids an alcohol free and drug free environment to grow up in.