I second the idea of using a cueing system to help him know when to stop/transition. My daughter loves music and read aloud books (on youtube, etc.). I use this approach before nap time. (To paraphrase Daniel Tiger: "It's almost time to stop, so choose one more song/story to listen to. And then, it's __ time.") Song plays and ends. "That was fun, but now it's done. Now, it's time for __." I think you could do this for shower time, too. Just offer to play 1 short song of his choice before the end. That way, he has a concrete idea of how much time is left, and auditory cues to tell him it's time to stop.

Have you tried the double-timer approach? The first time it goes off, that means there's five minutes left. The second time means that we're all done.

You could also turn the water heater down, at least until you figure out how to make it safer

I am finding this discussion valuable as well as interesting. My Adrian hates showers. He takes a bath and it lasts foreverrrrrr...

Maybe remind him when he gets in that when the water heater runs out of hot water, the shower water will go cold. I try to have an activity lined up for after and I tell him what it is - hey let’s take a bath and after we will have a cookie / watch a movie / call grandma. Sometimes he won’t get in the bath and I tell him the water will get cold and it’s his choice to either take a warm bath or a cold one. I am bluffing, I would not put him in a cold bath, but the connection between waiting too long and the water getting cold works