This can be so personal between couples that it's hard to give an answer, but the first thing I that popped into my head is when I read this was, "so what's wrong with his first name??" It's more persoanl than the nickname everyone else uses for him, but not overstepping your comfort zone either.

To me, "Sir" is a general term of respect for Doms I have regard or affection for, so if he met that criteria for me referring to him as Sir would be natural without implying anything more. At the point we became something more, I would ask Him to provide me with His prefernce for a title. With my J, "Master" was a natural fit.