Is he even eating in secret...? [Somehow, the topic of Sans's changed eating habits didn't come up in their talk. And so much else did, his head's still reeling. Maybe it's best they drop the matter of food, before it gets even more confusing.]

Well, you still have that opportunity. If you don't mind learning through being pranked. You could make a goal of it! Get him with some teleporting trick, to graduate.