I am going to apply for a postdoc call and I have commenced exchanging emails with a professor. In his first reply to my initial email he addressed me with my first name (Dear (my first name)). In my second email I address him as
Dear Professor (His first name only)
and his reply on my second email was
Dear Colleague.
Now I do not know whether I can address him with his first name without using the word "professor"?