My son's upstairs bathroom has been leaking downstairs for a long time. There is only a shower, no tub. Most of the ceiling below (in the kitchen) has been removed to look for leak. They can see water coming down, but can't pin point source. Leak is not so bad if standing in the shower, but if someone sits in the shower water pours into the kitchen downstairs. We were sort of assuming that maybe the leak was under the shower stall. I recently put a new toilet in his bathroom though, and now they have noticed that there is also some water leaking from toilet down into kitchen. It is not sewage water. It tells me though that leak is not limited to plumbing directly under the shower stall. The floor of bathroom seems to be cement board, so that would be a lot harder to cut through to explore from above. Any ideas on how to proceed.

With most of the ceiling removed, it should not be too difficult to locate the source of the leak.

I would suspect the shower drain.

Is this a tiled shower? Seats in tiled showers are notorious for causing leaks.They are usually installed poorly. Unfortunately, if you can't determine the source of the leak from what is exposed,you're going to have to take the shower apart.

This happened to us; the problem with ours was that the water pipe going INto the shower was leaking behind the plumbing wall. We tore the ceiling and bathroom apart trying to figure it out and ended up doing a remodel.

Once we figured it out, the solution was simple. Thread the pipe with white plumbing tape to the pipe supplying water to the showerhead and seal the leak.
Good luck with yours!
Salaam.