I am sorry to be the one to bear bad news...it is unlikely he can force the employer to give a job back. The reason is tied to the law in SC. SC is an "at will" state...In an "at will" employer/employee relationship, either side can end the relationship (the employer can fire the employee or the employee can quit) "at will".
As an "at will" employee your son's options are limited. His employer can fire for a good reason, or no reason at all. The only thing that they can not do is fire for a "bad" reason. A "bad" reason would be if they fired based on race, ethnicity, national origin, religion or gender some other protected class (like if he was a "whistleblower")
Here, if they fired your son because he left when told not to? He would not have grounds to sue.
This is the part of my job I don't like...when the law is not in favor of my customer. I wish I could tell you that he can sue but I can only provide you information based on the law so that you can act on the best available information to you...I wish I had better news, but can only hope you recognize and understand my predicament and don't shoot the messenger. I'm sorry!
Please let me know if you have more questions. I am happy to help if I can. Otherwise, please rate the answer so I may get credit for my work.