Get a small flashlight and let him play with it. Show him how he can make a spot on the wall. After he gets used to that, show him how he can put his fingers in front of the light to make a shadow. Finally, lay it on the floor and dance in front of it with him - showing him how you can make full shadows on the wall.
This may take several days, be patient.
In the meantime, can you rig a light right over the tub to get rid of his shadow? Or how about letting him take a bath in the dark or with a candle in the room (obviously, you run the candle!). If none of those work, sponge baths until he is happy with the tub again won't hurt.
at 4:55 PM on Oct. 21, 2008