GOD DAMMIT SEBASTIAN.

I need to bring in a stool sample for the vet tomorrow. The dilemma is that the cats share a litter box (well, two litter boxes) so I can't differentiate between the poop. I've been stalking Sebastian all day to try and catch him when he poops but I think he knows and is willfully not pooping BECAUSE HE HASN'T ONCE TODAY.

How do I get the little dude to poop?! I NEED HIS POOP.