When your ceiling is somebody else's floor, or where you share walls, you'll sometimes hear your neighbors. Hearing an upstairs tenant's shower running is something you accept - the drain runs through your wall - not something that's fixed by banning the upstairs tenant from taking showers. Yes, if they move out, odds are the next tenant will also take showers.
If he thinks the noises from upstairs are unreasonably loud, consider telling him to invest in a tape recorder and record them, asking him to also speak in his normal speaking voice on the tape while the sounds are occurring so that you have a baseline for comparison. (Otherwise, you're at the mercy of the volume switch.)

