I call my T by his first name. He has a Master's degree, so if I did use a title, it would be "Mr." I tend to call practitioners "Dr." when they are MDs and quite a bit older than I am. My T is only about 5 years older. Also, I have a PhD, and in my field, it is considered kind of snobby to want to be addressed as "Dr", even though yes, we did earn that title.