I do worry about his mental health, though even though you say he isn’t! He is severely brainwashed because everything he says is a classic talking point of the Incel community, and there is a massive divergence between what he believes, and reality. He may not actually be mentally ill, but he’s doing a great imitation of it! That state of thinking looks a lot like a Trump Supporter.

Im going to give you the name of a website that talks about stuff like this. It’s called
We Hunted the Mammoth, and the community there is very supportive, and sometimes have great advice dealing with people like your brother. They may have suggestions for how to get your brother back.