Hi, Samantha. I'm going to serve this straight up: As things stand right now, no you can not help him stop. What you describe in your question is an extremely selfish individual with nothing but excuses with no intention of stopping. Right now, he loves his drugs and alcohol more than you. There is no sense in deceiving yourself about this.
In addition, you are at risk. You may want to change him, but he would rather change you. Your partner would love nothing better than to turn you into a fellow addict, into someone he can get high with, who enables him. That's the way it works and it's not going to change.
It would be different if he sincerely wanted to change. It wouldn't be a picnic, mind you, but at least you would be in a position to offer moral and practical support. I very much sympathize with people addicted to alcohol and drugs. Addiction is a mental illness needing treatment. At the same time, I also sympathize with people who are the innocent victims of those with addictions. I've witnessed close up the devastation that people with addictions wreak on their families and loved ones. You do not have to be part of this. You deserve a better life. Trust me, your partner loves his drugs and alcohol way more than he loves you.
I know this is very hard on you, and that you have some agonizing decisions ahead. It's extremely difficult to break off a loving relationship, but when your partner is resistant to change there is no choice. Any decision you make is going to take a lot of courage. This is a time to reach out to friends and family.
Again, I hate to sound so brutal. No doubt you were wishing for a "yes" answer. But deep down, I suspect, you're seeking reassurance for the "no" option. You need to think of yourself first, and do what's right for you.
Finally, there's the matter of your own personal healing. This is a long term project. It starts with you asking yourself this: What is it about you that attracts "broken men" into your life? For instance, do you come from a family of addicts? We tend to repeat patterns. Do you have low self-esteem? Are you thinking someone with a drug-alcohol problem (and bipolar to boot) is the best you can get? Is your circle of friends addicts? (In which case you need to find new friends.) On and on it goes.
In short, rather than thinking about changing your partner, you may want to be looking ahead to changing yourself. More accurately, bringing out the best in yourself, working toward the kind of life that an obviously caring and loving person like yourself deserves.
The mere fact that you asked the question indicates you are ready to take the first step. Have faith ...