If I ever called him anything I would use his first name. Frankly I'm not even sure how to pronounce his last name. It has about a million letters, and I don't think he's ever pronounced it for me. I haven't had any reason to use his name to his face. He's called me by first name, so it seems fair.

I do think most therapists that I know personally, even if they are doctors, use their first names with their clients.