I had this problem 3 years ago. The doll didn't work for us, although I am sure she was grateful. In the end there was a long struggle and every so often I would tell him it had to be done, which he accepted, grudgingly. Easier with a boy, I suppose, with less to was.
But in the end, only about a year ago, I hit on something that worked, I got him to look at the ceiling and I held up some of my fingers and got him to count them. That wouldn't work on a 2 and a half year old but I wonder if you put only a tiny amount of water in the bath, like an inch or 2, and put sometihng on the ceiling, like stickers, might that work?