How can I get him to be more comfortable?

me and my boyfriend just started having and we both lost our virginities to each other. We've had 2 times so far and has never taken off his shirt even though i asked him to. We both have a mutual best friend and we were all over at his house the other day and our friend was like "did ya show her your 3rd nipple??" My bf just got this look on his face and i was like "you have a 3rd nipple??" and he was like "well, yeah... not really... its more like a birthmark type thing." I was like "aww let me see it!!" and he got all defensive and wouldn't let me. It really doesn't bother me and it shouldn't bother him, so what can I do?