Advice please.

Kind of long... Before I got pregnant, my boyfriend and I both agreed that we wouldn't want to know the sex of the baby until it was born. It wasn't until we were sitting in the genetic counselor's office at 12 weeks that I found out that he wanted to know. I refused to find out despite pressure from him. I wish I never invited him to the 20 week ultrasound because he looked when the tech told us to look away so he knew the sex. I was so upset that he didn't respect my wishes and our original plan that I cried. I told him I still don't want to know and he has been trying to convince me to let him tell me. I am sticking to my guns... until Wednesday. We went to visit his mom (who he told earlier) and she let a pronoun slip out. I am absolutely devastated and don't know what to do. I'm so upset that he in a sense ruined the surprise for me. I don't know what to do. I am so hurt and feel betrayed. Am I making too big of a deal about this? He doesn't seem to understand how important this was to me.  He apologizes for me being upset, not for his actions. I need to get over it because there's nothing I can do about it now, but talk about broken trust! 😣 What would you do?