Making my second pregnancy fun for the first born

I am a little nervous about telling my son I’m having another baby. He’s 7 and has been a little negative on the thought of having a sibling. He’s afraid the baby will be annoying and says he wants a sister over a brother. I can also see that part of him is competitive and is afraid to loose the affection of family members. We are planning to get him a chocolate cake when we tell him and have him and my in laws open up a box with an announcement.

What are some fun ways to tell him? Also how can I make my pregnancy “fun” for him?