The deepest need in this situation is not simply for him to break a habit, but for a transformation that only Jesus Christ can bring. An addiction is not a problem you can solve by sheer willpower, or by focusing more on a human relationship. The axis upon which every life revolves is our relationship with God. When that is out of alignment, every other relationship and struggle becomes skewed. His deliverance will not start with trying harder for you; it will start with faith in Christ.
A true, living faith is not merely words or a vague hope that things will work out. It is the substance of things hoped for, the evidence of what is not yet seen. If he puts his trust in Jesus, that faith will be accounted to him as righteousness, and the Spirit of God will empower him in a way no earthly resolve ever could. But a faith that produces no change in the life is a dead faith. If he truly believes, that belief will be demonstrated by works, a turning away from the old bondage and a new way of living. Pray that his faith becomes genuine, one that leads him to hate the sin that destroys him and to love the Savior who redeems.
You cannot be the center that saves him. Only Christ can do that. When his relationship with God is made right through faith, it will then begin to bring every other relationship into proper order, including his relationship with you and his relationship with those destructive substances. He needs to see that the drug is a master that will never let him go unless a stronger Master sets him free. That freedom is found at the cross. Encourage him to look to Jesus, who loved him and gave himself for him, and to not let old entanglements pull him back. Your role is to point him to that hope, to live as a witness of what faith looks like, and to trust God yourself for the outcome, even if the path includes suffering or waiting.