Also many children are afraid of the drain in the tub when they see the water going down. They have to learn that they won't also go down the drain and that other things like their toys will not also disappear down the drain. They don't have that knowledge yet that some things will go down the drain and disappear and some things are safe from going down the drain because they won't fit through the hole. They are not born with that understanding, they have to learn it.
I would try it for the rest of the month and see if he improves. You could give him a teddy bear or toy to keep with him while he is at school. Bring it home from school and let him have it before you take him in the morning, and let him keep it during school time and bring it home again. . A. B. Curtiss