Mi Shebeirach

Why do our nuschoaus for תפלות מי שברך refer to all males with the honourific of ר׳? I was looking through the text for the מי שברך for a child who brings his Wimpel to schul that Shabbos and it refers to הילד ר׳ פלוני ב"ר פלוני, which seems somewhat odd to me.

Also, how do we render ר׳ for the purposes of מי שברך. Is it רב or רבי? I assume that rabbonim receive a different set of honourifics and that the morah d'asra is referred to as מרה מורינו הרב