Have you tried leaving a f/t in there overnight?
Mine go through phases where they won't eat food if I offer it to them but they will chow down once I leave it in there and leave the room.

Maybe if you have extra rack space you can set up an identical tub close by and if he does go to the bathroom you can just quickly move him to the other identical tub.

My littlest one likes to tip water bowls and use the bathroom all the time so I have gotten in the habit of setting up an identical tub in case I can't clean it right away, and so he doesn't have to sit in his poo poo.