Kuri, a more subtle way to broach the subject and mabey get him to tell you is to try slipping a "well what does Sir wish for me to call him"? into the mix, make sure to practice it first so it doesnt sound forced

Amber is right as rain here, its a very personal thing for most couples. One way or the other the two of you will have to figure it out together.

It does take two to tango