If he never quite looked at it (why would he?) and generally calls you by your first name I would not be too offended; I'd smile and correct him at some point? I saw my T for many years before I got married and then not for quite a few during which time I got married and 8-9 years after seeing her again, she wrote a check out to me (reimbursed me for an overpayment) to my maiden name. This after 400+ checks written to her with my married name printed at the top? My style would be to say something like, "Hey, watch it, that's supposed to be _______". Or, "I think we've been seeing one another long enough that I can tell you my real last name. . ."

They see us once a week, us and 15-20-? how many others? Think of the first day of school and how elementary teachers use to butcher some people's names? The person corrected their pronunciation, the teacher made a note of it (or not) and repeated it a couple times to make sure they got it right and then they saw that child daily for the next 9 months, made out report cards, met with their parents, etc. It's your last name which they have never really had occasion to look at/study and which you never commented about; do you care about yourself and your T enough to set him straight? :-)