Hi K.,
It is a possible red flag that your son is this rigid about a sensory experience ending. He is very young, but if you have any other developmental questions later about your sons behavior, you should put this one in that category too and act sooner rather than later.
For instance, if in the next year you question his speech and language abilities, or if you wonder if his play skills are typical when he is three, then when you seek out profesional help for those things and you should mention this need for a specific sensory experienc and his extreem response to it ending. Children can display sensory warning signs much earlier than they display a full developmental issue. Don't panic about it by any means, but keep it in mind and act as soon as you have any cause to say "hmmmm?"
We also had this issue early, and it did turn out to be something we needed help with. Water was extreemly comforting to my daughter from a very early age, and we used it as a reward for good behavior and as a way to calm her too. We did try showers, and she stayed in these forever too, but it was not standing water, so I felt like I could read a book in the bathroom when she was little, or after she was a little older, I could leave her for a minute or two and she would be safe, just don't let him have anything to plug the drain with!
You could also try a water play table, if he is willing to switch to using water on his hands only in a toy durring the day, he might not need as much time in the tub at night. If he is seeking full body sensory experiences, it may not be enough. Once he is a little older, he may enjoy a large tub of dry beans or rice to play with for a sensory break, and if you are willing to put up with a little mess now, a wash tub full of corn meal gives them something to feel, and is harmless if they eat it. These things helped us, but she always was attracted to water, and still seeks it out for the sensory feedback she gets from it.
Hope all goes well,
M.
About me: Mom of three, age 8-15 (two with developmental disabilities) worked as educational advocate