I'd say it depends on individual interest. If he's wearing a suit or a tux then guess these go best (these are my personal selection) cause I don't think you'll go to KFC or Mc Donald's with a suit on :D
Show lots of legs to show that you like to be a sexy person but don't show cleavage cause that's too much slutiness. I am not a fashinoista so I don't know all the technical names lol but that's the best advice I could give you :) and don't be afraid to dress differently like to those cosplay girls that's super sexy ;)
What kind of restaurant is it? I'd say go with a nice fitting dress with maybe a thin cardigan or casual blazer on top. Or a nice pair of smart jeans, heels, with a nice (no logos or slogans) top or shirt, and a casual blazer. As you can tell I love women in blazers lol. So hot!
A nice dress (something classy if you have it to just above the knee and tights that fit the colour... maybe heels if you like that :D Just an idea... many other options! And enjoy Alex... it may already be in the past? Nice experience?