Its going to be really hard to keep that a secret. I have heard of parents getting into a fight when one knows but the other doesn't and ends up telling what the baby is and it escalates from there.
A good friend of mine have birth about 5 weeks Ago and her boyfriend had the same wish. She ended up not wanting to know because she said she totally sensed that it will be a girl! The baby shower was difficult since she told everyone it will be a girl as she thinks, she got mixed clothes, boys and girls and unisex. Then the baby was born and surprise it was a boy! She was completely shocked but still is a happy first time mom and now says she couldn't imagine having a girl. I am having the opposite experience. I could to wait to know and felt also 100% sure it's a boy but no, ultrasound revealed it's a girl! I was also super shocked at first and felt also a slight disappointment from my boyfriends family side! Because they all really wanted a boy! I m glad I know in advance because I am able to prepare myself better but I also realized it really doesn't matter it's just about having a healthy baby and giving safely birth. Just tell your boyfriend that you definitely are dying to know but you respect his wish wanting to wait!? Don't go crazy over blue and pink and telling everyone what it is because they might spill the beans without knowing. Make it into a fun secret! So he can be happy no matter what gender!