The mind is really the most devious sex organ. As in any bdsm relationship, if the Dom(me can control the mind of the sub, the body will follow. This is especially important in a cyber relationship. Obviously, since the Dominant can't reach out and touch the sub, mind control is of utmost importance.

Make him call himself demeaning names: needle dick, cunt lapper, slave-boy etc; make him write about a scene where the Mistress completely overwhelmes him; pretend to call in a second Mistress, one butt-fucks him while the other makes him suck her pussy; and like Oz suggested, make him go to FEMDOM websites, make him pay to join.

CyberDom is no substitute for the real thing, but if you're sub lives across the ocean, it's the best we can do.