That cheapskate! Don’t they have water at the deli?

Don’t worry, he likes you! The next time he comes over, just talk to him like you would anyone else. Say Hi, introduce yourself, chit chat, and ask him how about his work, how long has he been working there, does he like it, where does he go to school, how many kids and ex wives he has, child support etc. Ok, OK... just kidding on the kids and stuff.
If he orders a burger the next time (don't forget to say, "You want fries with that?") and sits down, time it and try to get out from behind the counter if you can and pretend to go and wipe the tables down and talk to him. Be assertive without intimidating him and do your own talking. Don’t ask anyone else or let anyone else do the talking for you. It's a sign of confidence if you do it yourself.
The key is to just talk to him like you would any of your other friends.
Report back any positive progress.