Am I wrong in assuming he watches a lot of political content on Youtube? Far-right radicalization on Youtube is something that has been happening a lot these last 3-5 years, and honestly I think it's something you'll have a hard time dealing with on your own. It might be worth trying to get him to watch Youtubers that explicitely aim to debunk the Far-Right. For example, there's a German historian named Three Arrows who recently did a video on the Youtube radicalization process. Contrapoints and Shaun are also channels you might wanna look into. Check out /r/BreadTube for more ideas. Good luck.