Just don't argue. Refuse to engage. It sounds as though you've told him you don't want argue. That's good. Now it's time for you not to play his game. Discuss things that are not argumentative.Stay far away from political discussion. When you discuss the gospel do so without entertaining debate. Apologetics is great but it is not your job to answer all his objections. Allow God to do that. Above all, pray for your brothers salvation.