@Water bc daddy is used in son/dad play and master is used in master/slave play. If they arent into that, why would they use them? Sir seem to fit well for me, as I see it used in manhwas a lot. 
 at least he didnt go with Hyung. O.o adjussi would have worked but that legit is specific for middle aged men.

Why dony he just call him daddy or master is more appropriate than sir. Like sir seems to refer to a...