Most Helpful Guy
From a guy's perspective:
The #1 thing to go for is comfort. No need to show up in a dress & heels lol, esp. since it's a birthday party lol.
Now for the "looking cute" part. Pretty much, guys wanna see you wear something that flatters your body. *licks lips* You're a girl that's >= 18yrs, so I know you have something comfortable that makes your body look good.
Pretty much...don't overdress. A pair of jeans/leggings, a "cute" top & comfy shoes is all that's needed. Don't try so hard; he already seems to like u. :)