I, oddly enough, actually think “Father” would be the proper response here because I think it’s a liturgical formula. Much like when the deacon tells the priest “Jube, domne, benedicere” (Command, lord, a blessing). Lord isn’t a normal title of a priest (typically, if used at all, it would be for the episcopus dominus) but in that situation it is the prescribed address. Somewhat similarly, even though one would normally refer to a bishop as Your Excellency, in this liturgical setting (even though confession is private there is still a rite) it seems proper to use the same formula, “Forgive me, father,…”