That depends on who is speaking. Most servants will never use a name, only "My Lord/Liege" and keeping their gaze downcast at all times. An experienced valet or butler can get away with "Master" and a brief meeting of eyes. Peers can say "von Mattian" or just "Mattian", friendly peers can say "Cosmas" in informal conversation. Individuals of higher rank will usually address him as "Sir Mattian", unless on extremely good terms and thus liable to use first names in informal conversation.
You, as the narrator, should probably use either one of "Sir Mattian" or "von Mattian".