The Bible does warn us about the danger of getting so far from God that we'll no longer be able to hear His voice. It says, "A man who remains stiff-necked after many rebukes will suddenly be destroyed -- without remedy" (Proverbs 29:1). These are solemn words, and we must take them very seriously.
But does this mean God can't break through those barriers and bring someone who has ignored Him to Himself? No, not at all. I often think, for example, of King Manasseh in the Bible. (You can read about him in chapter 33 of the Old Testament book of 2 Chronicles.) King Manasseh has been called "the wickedest man who ever lived" -- and with good reason, because he not only hated God but embraced every type of evil, including witchcraft and sacrificing his sons to the pagan gods. He even banned the worship of God and placed a pagan idol in God's temple.
But one day King Manasseh was taken captive and placed in a foreign prison -- and there he turned to God. In spite of his countless sins, God forgave him -- and in time he was restored to his throne and spent the rest of his days trying to reverse the damage he'd done. The same can happen to anyone who truly repents and receives Christ.
Pray for your brother, and urge him to turn to Christ before it's too late. God's Holy Spirit can do what we can't do in someone's life, and I pray this will happen to your brother.