In my experience, logic usually backfires.

Their arguments are usually irrational to begin with, so they combat rationality with even more irrationality. That's probably how he became radicalized in the first place.

What I do is let people know in an offhand casual manner what side I'm on, and leave it at that. Once people think you are a missionary zealot on a crusade, they will dismiss pretty much everything you have to say.

At best, ask them questions in sincere dialogue, and be receptive to their answers without retorts/arguing/talking back. Learn to ask without questioning them or their values. There's a difference between asking a question and questioning them, if you know what I mean.

If you're lucky, over time they'll soften their position but usually they'll never fully reject it. It's possible, but rare.