I'd check the plumbing, first. Specifically the velve. It might be that you had the water temp set on way, and your son had it set differently, which means water would be coming from a different source (the hot supply vs the cold supply).If that's not it, then most likely you're right. That pan's going to need to be replaced. I can't say for sure, though, without looking at it.

Bill,

I did have the water on cold when we were testing this. It seemed silly to use hot water. But, the water isn't leaking in the area where the valve is and water pipes go up the wall. That area is dry, it is coming out of the corner of the shower floor area.

It just seems really next to impossible to find a water leak from underneath the shower when there's subflooring. I can imagine that subfloor can be removed without compromising the stablility of the floor area right? How would you fix it - the subfloor would have to be replaced in pieces.

If the pan does need to be replaced. Can it be done by removing one row of tiles around the floor, or does the whole thing need to be redone?

First thing I'd do is flood the pan. If you think it's the pan itself that's leaking, then I'd plug the drain and flood the pan with water, to just below the height of the curb.

If it stays dry your pan is fine and it's time to look at the walls and what's behind the walls.

Is water spray that ricochets off the bathers' body getting behind the valve trim and into the walls?

It almost sounds like water is getting behind your tile, and it was trying to get back in where the wall meets the floor.

The water saturated your caulk, which if it's a latex caulk, will stay mushy if continually exposed to water.

When he sealed this joint tight, the water had no where to escape so the water eventually backed up high enough to get into your framing bays and drip down to your kitchen ceiling.

Is this the same one where we thought you might have a plumbing leak inside your wall? Did you have the saturated and swelling shower curb?

If this was an acrylic base, then I'd say yes, the weight of your son could cause a leak that might not occur were a bather not standing in the shower. But with this being a solid tiled floor...I'd say no.

Depending on the age of the valves, it could be what Bill wrote. The packing or stem of the valve has come loose a little bit and it's dripping. Example, if the hot water side is leaking and your tiler only ran water from the cold water side, then it never would have leaked. That's a stretch, but worth looking at.

Can you pull the valve trim kit so you can see the valve body and run the water? Sometimes you can wad paper towels up, wrap them around the pipes or tuck them under the valve body where you can't see or touch, and see if they become wet.

I've also seen leaks happen depending on what spray was used on a multi-spray shower, some sprays cause a little more back pressure in the plumbing, that can cause a leak at the arm, or at a sketchy soldered fitting.

One last thing, the door. Does the door, it's hardware, or trim, funnel water in any way shape or form across or down the wall?

Many possibilities, all frustrating to say the least.

Mongo

Update...My BIL is a remodeler guy from out of town, and he is here today taking a look. He has filled the pan with about 4" of water, and so far no leaks.

He did find one corner of wall caulk that was damp and suspect to be damp since the shower hasn't been used since Wed. afternoon. It's just in the opposite corner of where the leak is happening, I know water travels, but still seems like alot more water coming out than from a small corner leak.

He also meantioned the hand held or valve, so sounds like he's on the same wave length as far as trouble shooting this!

Thanks for your input, I'll keep you posted with what we've done and found out.

Well the pan didn't leak when we filled it about 3" full of water. However, the liner did fill with water and leak when we used the shower head against the one corner of the shower by the door. My BIL is amazed at how much water is leaking in to the pan liner from the shower spray as the joints appear to be very solid - no cracks, silicone looks ok etc.

There has to be a leak somewhere obviously. I am thinking maybe the floor tiles hold standing water, but when when the shower spray hits them water must be getting through somehow.

Anyways, I'm thinking we should at least regrout the floor and redo the threshold to fix this problem. Do you think the floor tile, cement pad and liner all should be redone?

@HU-916977380 it's probably better to start your own thread with a complete explanation (photos if you have them) - your situation may seem to be similar but there are a lot of things that cause leaks.