Success!

Unable to sleep, I heard him moving around and stalked him into the bathroom. Alas, I was not sneaky enough so it was me who went to the bathroom. He remained in the bathroom though, so I put him on the sink and turned on the water (he likes to drink from the faucet). He didn't drink since he is not feeling well - But, as luck would have it, he couldn't hold it then and went right to the box.
Yay!