Your son should go pick up an AAFES AT&T int'l phone card. They have the best convesion rates but make sure it's an int'l phone card. I'm sure that there are other int'l phone cards besides at the PX if you want to mail him a few.
Also I don't know how long he has been there or how long his tour is but he could look into either getting a prepaid cell phone plan. I know that the soldiers buy the phone off someone about to leave and just take that # over.
Some others I know have dsl internet in their barracks room and then have vonage hooked up to that. If you have AT&T or some other long distance plan you should be able to add Korea to your long distance for very little expense and then you could call him.
