“Do you want to do this again?” sure sounds like asking for a date, especially if he paid for sushi for you.
This is the classic, and IMO polite, way to do it. Don’t make it obvious, just do it in passing. Gets the message across if necessary, while making nobody uncomfortable (which would probably be him if it is necessary, you if it isn’t). That’s assuming there really is someone you can mention; you can also mention someone you’re just interested in, which usually also gets across the message that he’s not in that category.
Which isn’t to say that you should be afraid of being straight-up and honest about how you feel/think about him, just that there’s no reason not to try the subtle approach first.
But regardless of how you work that, you should probably also be honest with him that you really can’t have him hanging out at your booth for any length of time, as even though the company is nice, you need to get work done.
and in fact I did not eat $20 worth of food (the price of the item he “bought”). Still, I think I should have seen something like this coming and been prepared for the “tough” questions. But I always have a hard time in situations like this. Just do a search. You’ll see. 
) and just manage to not be available to hang out the next couple of times that he suggests it—He will get the hint and will still be able to maintain a bit of face when you interact at work.