Tell him bluntly that this needs to stop. Time is being wasted as people trek across the building so they can do something as basic as wash their hands. You are scrounging pens wherever you can find them--and that takes time.
Also, this is very unsanitary. What will end up happening is that either people will use that restroom and then walk to wherever there is a sink with soap, spreading germs along the way, or they will go directly to another restroom that does have soap, wasting even more time. A third possibility is that the soap will be used but diluted, interfering with the ability of the soap to handle germs.
A fourth possibility is that people will just use that restroom and then not wash their hands at all....which does not bear thinking about. Unless somebody has a skin condition that means s/he needs to use their own special soap or none at all or whatever, not washing hands at all after doing one's "business" is just gross, IMHO.
Maybe you could find one of those flyers on the internet or somewhere that details how you are supposed to wash your hands to avoid spreading germs, and post it in the restroom. That will call people's attention even more to the fact that you all actually need soap. Cold and flu season will be starting in a few months and productivity will go way down if everyone is out sick with something or other. It will also be expensive to the company if you all get paid sick leave.
And to refer back to another part of your comment that I snipped for length, imagine how bad it would be if word got around that the people in this department struggle with being able to wash their hands properly. I don't know if it's a situation where other employees could then avoid you completely, but at the very least this would make them reluctant, and affect morale.
Bottom line, I think this is a classic case of "penny wise, pound foolish" and until it is really driven home how many of those proverbial pounds are being wasted, this silliness will probably continue.
ETA: Aquamarine also made good points about this subject.