I have an in-law family member who hero worships Charlie Kirk. He wears the T-shirt and has a Charlie Kirk sticker

[ForgedCrank]

9. You handle

it by keeping politics to yourself and do nothing as long as he isn't harassing you about Charlie Kirk all day. If he does, kindly let him know that you'd prefer to not talk politics or religion for the sake of civility. If he still refuses, simply stay away form him because lashing back is the same thing, don't be like that.
You won't be changing his mind and if he's treating you decent, you do the same because that's what decent people do.
As far as what I would think of him? It wouldn't matter to me because I recognize the right of others to have differing opinions and I don't hate them for it. I may secretly think they are wrong on some things, but that is irrelevant to them and not worthy of a family argument.
Anyway, that's my advice.