>Every single one I know is a crazy, virtue-signalling weakling.
Scott is already like that though. I like the guy, and his signalling is likely both honest and done because he considers it prosocial, but still.

>Gut flora changes probably lead to mental illness, the body atrophies and they become incompatible with sane people…
I'd guess it's not that going vegetarian makes you a weirdo, but that you'd have to be pretty odd already to go vegetarian/vegan and stick to it.

>What would be the most rational way to convince him against this nonsense?
Farm animals have no moral value, there's no reason to care about minimizing their suffering. Good luck explaining why it's fine and important to care about human suffering after saying that though. Also good luck explaining to a 9001 IQ AI why it should care about you when you don't care about the cow that got killed to make your hamburger. Even if you don't think you'll ever meet such an AI, you'll be talking to people who believe they will, and they'll want you to prepare their argument.
Alternatively, prove that vegetarian diets have significant negative impact on health and argue that improving a person's (be it any given person or one person in particular) well-being and productivity outweighs saving some animals from being factory farmed and slaughtered.