To be quite honest I don't think that there is any specific protocol that you have to follow, unless it was otherwise stated.
He is not your Dominant so you are not obligated to follow any certain criteria as you do with your own.
If you feel more comfortable in calling him Sir or whatever than that is not a problem to do so.

You are just two people who know each other and are meeting. You undoubtably are going to be respectful of who he is, but other than that you would treat him as any other person you are meeting for a drink, dinner, coffee etc....