You could try to bring him back, its a chore and there is no guarantee you can push someone back from extremes in ideology, but its worth a try.
I had an older cousin that was starting to lean reactionary because he has what a lot of people have throughout their adolescence and young adulthood: an inherent suspicion of authority, and with that, any information that comes from sources close to authorities is suspect.
Its what leads so many people to pick up information from grifters saying "This is what THEY didn't want to teach you in school."
I got him listening to some Kwame Ture and sent him a few books he started reading - and he's recovered a bit