That does seem odd to me, too. Most of my professors don’t/didn’t even want to be called “Professor so and so”; they have told me to call them “Mr./Mrs./Ms./Miss so and so.” Sometimes, they even like just to be addressed by their first name.
I know that it seems tedious and odd, but you must respect his wishes because he will probably continually remind you and eventually get annoyed that you don’t address him properly. Such a formal salutation is usually used only in the initial email, however.