'Mister' would be unfamiliar to him as well, before travelling with the Doctor, but he's been introduced to that one. The problem is that everywhere they go, the customs are different, so anything like that is a lost cause to keep track. And Adric was pretty rude by even his homeworld's standards. (He's gotten better. Mostly.)

"Wait, Fontaine was your surname, wasn't it?" he asks, suddenly realizing. Adric's not that used to surnames either. It wasn't until he met Tegan that he was introduced to the concept, the way people from Earth use it.