This is a tough question and while I totally understand you wanting to get some advice, it's important to know that there is no 'right' or 'wrong' answer here. The situation calls for you to make a choice based on how you feel. For someone who spends a lot of time trying to spare the feelings of people around them this can be the hardest thing in the world. But it's true. You have to decide what feels right and makes YOU happy.
That's what I did and can say I have no regrets. From the second I found out I was pregnant my son's name was picked and ready to go. True, there was tons of fighting with loved ones who didn't like it but in the end it was me pushing D'artagnan into the world, not them. So I felt justified in my choice. The world kept on spinning and with time everyone learned to except my choice because no matter what they had to say on the matter, it was happening.
That's just what worked for me in my situation. What ends up working out for you could be the same or different, just do what feels right and tell your ex to back off a little. There are way more important things to be arguing over then the name. You guys are getting ready to bring a baby into the world after all. :)