While the military services have professional cooks, they do not have a job specialty for the people who clean up the dining areas - in civilian terms they'd be called busboys, dishwashers, etc. Therefore, many of the junior military people spend a few months on special detail away from their regular job specialty augmenting the kitchen personnel in performing those duties.

For the OP, most of the airlines (UA included) will make exceptions when a military member's orders change. Problem here is your soldier/sailor/airman didn't have his orders changed - he had his leave plans changed. Nonetheless, if he calls and couches it in the "my orders were changed" he may get a break...but also may need to provide proof at check-in, in which case I'd suggest he bring copies of his original leave papers and his revised leave papers to see if that will work.

Edit: If the military member already had approved and signed leave papers by his chain of command, I'd suggest that he elevate the issue upward through his chain of command, explaining that he purchased non-refundable tickets based upon his approved leave papers and that changing them will cost him personally.