I don't talk about politics with any of my friends, really. If I do, I usually do what you do and I sit on the fence. I do that, or I try not to comment back. (There are only a few issues I'll actually discuss.)

Don't tell him you had a melt-down, but tell him that this discussion is triggering and you'd like to avoid political discussions in the future. He should respect that line if he really respects you. If he responds and says something questionable, maybe come here and we can try and help you understand where he is coming from to maybe prevent a meltdown?

My best friend and I have opposite political beliefs. We have decided not to avoid politics for the most part because we argue and there is a huge potential of someone getting hurt. It works for us.