My husband had a business partner for 6 years and that partner embezzled money etc. then pushed him out of the company. We know the bible says not to take your brother to court. If it boils down to if he’s a Christian or not, that is questionable because he holds to a works-based belief.
Paul wrote that it’s better to be cheated than to take a brother into court, that matters between Christians should be settled within the church. (1 Cor. 6:7)
In Matt. 18:15-17 Jesus instructed us to first go to a brother who has wronged us directly and try to settle things. If that doesn’t work then take him before the elders, and if that doesn’t work to treat him like a pagan.
If you and he belong to different churches, ask him to set aside your theological differences and choose one or two from each group to listen to both your stories and render a joint opinion.
If you still can’t get resolution, or if he’s unwilling to cooperate, then I’d say you’re free to pursue a legal remedy. He will have demonstrated that he’s unwilling to be treated as a brother and help you find a remedy that honors the Lord.