I disagree with finding out to keep it a secret because that won't work, of you buy something blue or accidentally refer to the baby as him for example then the secret is already out.
Maybe you need to have as conversation about both of your reasons why and go from there, to be honest the surprise will be the same during pregnancy or in labour, as you are still finding out the same thing.
I never wanted to find out what the sex was with my first but my bf did so we found out and I always said that any baby after her would be about surprise, but we found out what we are having this time because it made it easier to get my daughter involved in the pregnancy and helps her understand that it's a real person. (They were my personally reasons for finding out). I'm sad that I won't have a surprise but it's alot easier to get everything ready.
Tell him that since you want to find out you will and just keep the sex a secret from him .