Unless it comes out in the Court Martial that order was illegal. Or in this case issued by an ineligible national command authority.
The Army takes a dim view of soldiers who decide for themself what order is valid and what is not. His company, battalion, brigade, and divigion commanders all will issue him legal orders. He disobeys them at his own risk.
Now if Obama ever gives him a direct order then he may have a case, but I kind of doubt that will happen.