I am praying with you for this conversation. What you are asking for is not only the right words but a heart that is submitted to God. True wisdom is not found in crafting a convincing argument or managing the outcome. It is hidden in Christ, and the fear of the Lord is the beginning of that wisdom. Ask Him to put His reverence in you before you speak, because a right relationship with Him is the axis that keeps every other relationship in balance.
As you prepare, do not look for the excellency of speech. The wisdom of this world knows how to win an argument but not how to heal a brother. The wisdom that comes from heaven is pure, peaceable, gentle, open to reason, full of mercy, without partiality and without pretense. That is the kind of wisdom you want governing your words. It will make you sincere, because there is no hypocrisy in it. It will help you speak the truth in a way that does not curse or wound the brother made in the likeness of God.
So pray not only that God would give you a strategy for this conversation, but that He would make you a peacemaker in it. Ask Him to show you anything in your own heart that needs repentance before you try to set anything right with your brother. Then you can speak from the Spirit and not from human calculation.
I also join you in asking the Heavenly Father to work in this brother's life. Leave the changing of his heart to God. You cannot make him submissive or obedient, but you can trust the Lord who works in him. Pray for him to hear God's voice, and trust that God may have a different way forward than you expect. He is not bound by our patterns. He gives wisdom and He gives peace.
Let us ask Him together: Father, give this conversation over to You. Grant sincerity, humility, and wisdom. Guard the tongue. Bring healing where it is Your will, and make both hearts ready to obey You. In Jesus Christ's name, amen.