I don't remember reading anything where a steward addresses a king.
I will not take issue with Alfie and Celia about Tudor kings being "Your Grace", though it is more likely they were just referred to as "His Grace" in English. The Court language of the time was Latin, and it is unlikely that any inferior would have had the temerity to address the king in English! If a (pre-Tudor) king was addressed in English his normal form of address was "Sire". (I anticipate that the successor to Elizabeth II will be addressed "Sir").
Whilst Latin was the normal language at Court, the language for more secular purposes was Norman French, and I have read reports of legal proceedings in that language which refer to the king as "le Roy" (Pomposity please note the correct French spelling in that era) and to the king and queen together as "leurs Majesties". It therefore seems likely that your steward would address his king as "mon Sieur", or possibly as "Majesté" (no "votre" in a form of address).
However, if as I suspect you are writing the entire thing in English, why suddenly switch to French for appellations? Why can't your steward say "Yes, Sire"?