There’s no reason why he can’t call him Darth.
Because Darth is not his name and he’s never called him Darth before or since and no other Sith is addressed as just Darth. That’s 3 reasons.
Let Ben have his moment of fun mockingly calling his forner padawan with a Sith title. It’s in line with Ben character. Like Luke calling the Emperor “your Highness”: it’s a bit sarcastic.