Thank you for sharing this burden for this brother and for your honest heart in bringing him before the Lord. Your care for him, even while you feel fear, is itself a reflection of the love of Christ, and it is right to place him in God's hands.
We will pray for him and continue to do so. What you describe is a man who has truly passed from death into life. By being born again through the Spirit of God and being baptized into Christ, he has been brought out of the old realm of darkness and death and into the kingdom of light and life. That which is born of the flesh is flesh, but that which is born of the Spirit is spirit, and he is now a new creation. The same Spirit that raised Jesus from the dead now dwells in him.
Scripture teaches that there are two laws at work. There is the law of sin and death, which is the law of our fallen nature, and there is the law of the Spirit of life in Christ Jesus. The second is greater and sets us free from the first. It is the greater power that expels the lesser. The kingdom that once held him is marked by darkness and death, but the kingdom he has now entered through Jesus is marked by light and life. Death no longer has the final word over him, for the gift of God is eternal life through Jesus Christ our Lord.
This does not mean the struggle is over. There is a real war between the flesh and the Spirit, for these two are contrary to one another. The mind set on the flesh brings death, but the mind set on the Spirit brings life and peace. He will need to learn, day by day, to yield himself not to the old patterns of the flesh but to the Holy Spirit, and through the help of the Spirit to put to death the deeds of the body so that the Spirit may be dominant. This is a choice that is now his in Christ, to walk not after the flesh but after the Spirit.
You mentioned feeling that the spirit of death is in control in his life. We understand why you would feel that way given his past of violence and addiction, his time in the hospital, and his present lack of home, work, and Christian fellowship. Yet the Scripture gives us confidence that if the Spirit of God dwells in a person, he belongs to Christ, and the evil one cannot simply take hold of him. The whole world lies in wickedness, but those who are of God are kept. God's Spirit bears witness with our spirit that we are His children, and as children we are heirs with Christ. That witness brings assurance that he is now a son of God, not an outsider.
For this reason we pray that God will establish him in that assurance, that he will know the joy and peace of walking in conscious fellowship with God. We pray that the Lord will deliver him fully from every hold of darkness and fill him with the life of the Spirit, conforming him more and more to the image of Jesus.
We also pray for very practical needs. He needs a home, he needs work, and most urgently he needs to be surrounded by believers who can walk with him, teach him, and encourage him. To be alone in a worldly environment leaves him vulnerable and without the support every new believer needs. Pray with us that God will provide godly fellowship for him, a place where he can be taught the Word and learn to live this new life in the Spirit.
Do not let fear keep you from loving him in prayer. God is able to keep those who are His, and His Spirit is sufficient. Continue to bring him before the throne of grace, trusting not in our strength but in God's ability to be his sufficiency.
Let us pray together now: Father, we thank You for Your mercy in drawing this brother to Jesus and giving him new life. We thank You that Your Spirit now dwells in him and that You have made him Your child. We ask in the name of Jesus that You would guard him from every work of darkness and death, and fill him with Your Spirit of life and peace. Teach him to yield to You and to walk after the Spirit. Provide for him a home and honest work, and set him among believers who will love him and help him grow. Let Your Spirit bear witness to his heart that he is Yours, and keep him in Your light. We trust him to Your faithful hands, through Jesus Christ our Lord. Amen.