Bluntly, I’m not sure why this a conversation. I’m all for productive disagreements (even on small things!), but there’s not a substantive point being argued here.

Try being more blunt & concise. Instead of explaining, just say “please add unit tests and I’ll approve,” and then disengage. They’re the one who needs their PR approved - not your problem.

The key here is escalation path. He’ll have to bring it up to his manager, and then the conversation is with his manager, and then you can have a broader “he can’t just ignore every standard” conversation.