I think the Bible makes it clear in the book of Mathew on what to do on such an occasion.
Here is a quote:
“If your brother sins against you, go and tell him his fault, between you and him alone. If he listens to you, you have gained your brother. But if he does not listen, take one or two others along with you, that every charge may be established by the evidence of two or three witnesses. If he refuses to listen to them, tell it to the church. And if he refuses to listen even to the church, let him be to you as a Gentile and a tax collector.
Matthew 18:15-17 ESV
This is the step suggested by the Bible - Speak to him, reason with him, if he is reasonable, then you have won the battle but if he refuses to be reasonable, then seek counsel, bring in two or three witnesses to speak to him; if he refuses yet again, then tell it to the church: cry out for help. And if he refuses to listen to the church, then let him be.....
but I will just add, continue to pray for him, love him still but do not stop been good to both he has declared as enemies and to himself.
I hope these helps.