Dress Calamity!

Okay so I have my first ever date coming up in the next couple of weeks and I have found an amazingly sexy off the shoulder black bodycon dress. I am a size 18 but I hate having to wear frumpy clothes because of that! I have put on the dress and my belly is really emphasised and even I can admit it's not going to be a good look for me, especially when we're making out in his car and my bowling ball of a belly is sticking out! 
Has anyone got some ideas for underwear that is sexy but also will help me look slimmer? Just in case for whatever reason the chance comes about we do get to second base and he sees. 🙈