Welcome. I have some bad news for you. You can't help him quit. There's nothing you can do to cure an addict of their addiction. Read Co-Dependent No-More by Melanie Beattie. It will help you learn how to step back from trying to control his choices and focus on your own choices.
Lots of addicts SAY they want to quit. But words mean nothing. It's about his actions. You can't quit for him. He has to do it himself. He has to do it because his life sucks and using sucks. Not because you tell him to quit or because you threaten him he has to quit.
If he wants to get better he'll do it all on his own - he'll quit hanging out with drug addicts and seek out people who are in recovery from drug addiction. He'll do what they tell him because they have something he wants.
I think this is a good time for you to step back and see what he does next... because you didn't cause this. You can't control this. And you can't cure this.
If you need more proof of that... read the stories on this site. And believe them.