I wonder if the best solution to posts like his is to simply ignore him. The alt-right/trumpkin playbook is to never, ever go on the defensive. Never apologise, never concede anything, never admit someone has a good point. Just deny, lie, and play stupid. Pretend Obama's 18 lies over eight years are somehow comparable to Trump's habit of lying that many times in a single day. Stall the whole discussion, or better yet, try to derail it to be about Obama, AOC, or some other boogeyman of the alt-right. If someone on the left does something wrong (because everyone who disagrees with the alt-right and/or Trumpkins are "on the left"), pounce on them even though it makes you look ridiculously hypocritical. Then accuse "the left" of hypocrisy, just to make everything even more absurd.
They are effectively trolls. A new kind of trolls, but still trolls. Just let them yell into the void. You'll never convince him anyway, and he knows how he's coming across, he just doesn't care.
Once he realises he can't derail discussions anymore, he might get sulky and put a red cross in his avatar, but I for one won't even see that, as I have him on ignore.