I've never had much luck guys so I feel dumb for not knowing how to deal with this situation.
Most Helpful Guy
1) watch him a bit, does he do that with every female he waits on? it may just be his way of getting a better tip. (assumes US based)
2) go with it if you're comfortable. Maybe he'd be good practice for you in a safe place. Practice makes perfect.
3) enjoy it without reciprocating. Nothing wrong with just enjoying the flattery and moving on.
4) tell him enough is enough -- in the nicest possible way. Talk about school, graduation day, prom, anything to let him know you're under age. Do it in a flirty way even, tell him that if 16 wouldn't get him 20, you might be interested.
Most Helpful Girl
Try bringing up somehow in casual conversation that you are underage, that way it isn't so awkward. Instead of coming forth and just telling him that you are underage, talk about school maybe. This will at least let him know that you are in high school. So far, though it seems as if his intentions are harmless. If that changes and you become uncomfortable with it, let him know and then if it still persists after that, either stop going to that restaurant or let your parents know what is going on.