Sending him an email or changing his password is a good idea. It doesn't really cause any harm, but even thick headed people understand how it could after you show them what can be done in their absence. Quick story, very similar.
I had a guy who never signed his reports at the end. I reminded him several times. Finally, I just re-wrote his report. Put in all kinds of crazy stupid things. Printed it out and had it sitting there waiting for him when he came in.
I asked him what was wrong with his report and he said "Oh, I didn't sign it." Then he signed it.
I politely asked him if he had read what he just signed and his face went blank when he looked at it. Like he couldn't figure out why he had actually written that stuff. I just smiled and told him, "This is what CAN happen if you don't sign it."
He never forgot again.