Mine called my husband by the wrong name in the middle of a session after 7-8 years. I'm like, "Who's Steve?"

If you are just starting to see the T and neither of you know one another very well yet, I wait a bit, see how he is once you all get settled into working together. If you haven't said much, there's not much for him to hold to? If he does it again, I'd joke and say something like, "It's Suzzie, Mr./Dr. Wrongname" (make up a name for him).