What you describe actually sounds kinda hopeful. He may not be sure what to believe, and is trying to find direction in people who seem like they know what they're talking about because they act like they're extremely sure of themselves. But it's all just an extremely destructive con-man game attempting to justify the speaker's own fears by getting others to join them in their thoughtless blind hatred.

I have no idea how correct this notion of mine is (and this is the first time I'm expressing it to someone other than my wife and adult son - yes, that makes me an older male so maybe there's hope for yours, too), but I've explained the existence of trans people to myself by remembering back to high school biology teaching us that life is constantly mutating because genes often don't get copied correctly. People evolved from such mutations, get their appearance, health issues, intelligence, etc. from such mutations. As such, it's perfectly logical to me that such mutations happen wherein the brain winds up with a different gender than the body manifests. Again, I don't know how true to life that thought process is, but hopefully it can help you explain to him just how trans people might come to be.

Anyway, I wish you luck! 🫂

ETA: I know that makes it easier to single out trans people for ostracization as "mutants," but the fact is we're ALL mutants or we'd still be amoeba swimming in the oceans.