It's really hard. But you have to listen and gently push them in the right direction. Don't endorse, but don't outright dismiss him because then they will never listen to you. Listen and be honest. Let him do most of the talking and throw soft disagreements to try and add perspective when the trust is established. A lot of fascist rhetoric uses anti-capitalist rhetoric to disguise itself as beneficial to workers. Try to slowly make him realize that the real enemy is capitalists in general as opposed to whatever Jewish elites he thinks are the enemy.

Encourage his anti-capitalist viewpoints and lightly discourage his fascist bs. Ask contradicting questions in a charitable tone, to poke some holes in their arguments. Not to the point that they get defensive but just to sprinkle seeds of doubt in their mind. This is the only effective way I have found. If this were anyone but family you would want to snap and show them that they're stupid and delusional, but that isn't really an effective tactic at opening a narrow mind.

My older brother has autism and used to find refuge in the manosphere types. The jordan petersons, Isaac Butterfields, the "feminists getting owned" types. I think, or at least hope that my gentle approach was part of the reason he has abandoned these figures and opened up his viewpoints. Nowadays he talks about "the rich" much more instead of feminists or vegans or whatever culture war scapegoat of the day. He's far from reading Marx or putting a Lenin quote in his bio, but he's readjusted who he should be directing his anger towards.