For me personally I don't understand why people find out the sex. I felt like it was one of the last surprises life would give me as an adult (as a child they happen all the time and I lurve surprises)
We had a boy first then a girl second. Now pregnant with our third this is still going to be a surprise, I have everything for both sexes anyway so no big deal!

Also you can make it a bit of fun by having bets on the sex and doing all the little wives tales to try and tell!!
It would be hard if both parties can't agree on what they want. Good luck with trying to convince him