Scott's expression is just sheer amusement. Kate's totally drunk and she's totally goofy and he can't remember when's the last time he went out with anyone that just weren't a few mellow dad-workbuds.
This was taking him back.
But there comes a point where she draws closer to him and he draws back, and Scott's not sure if it's because of Hope or because Kate's obviously drunk (or getting there) or a combination of the two.