I am a law school graduate. What I offer is mere information, not to be construed as forming an attorney client relationship.
Yes, but be prepared that he can lose this job as a result of the legal recourse.
They cannot just accuse him/imply something was his fault without notification the same day. It doesn't sound right to me, and I think that because he is 15, they believe they can get away with it.
Let your son stand up for his rights. If he is legally able to work (work permit, etc), then he should have no problem bringing up a potential libel lawsuit. See, read the following definition below and you will see why I say libel. They are accusing him in his profession/work place of underhanded/fraudlent and criminal activity. Employers can't do that without being able to substantially back it up with hardcore evidence.
Defamation, sometimes called "defamation of character", is spoken or written words that falsely and negatively reflect on a living person's reputation.
If a person or the news media says or writes something about you that is understood to lower your reputation, or that keeps people from associating with you, defamation has occurred. Slander and libel are two forms of defamation.
Libel is a written defamation.
In order to prove defamation, you have to be able to prove that what was said or written about you was false. If the information is true, or if you consented to publication of the material, you will not have a case. However, you may bring an defamatory action if the comments are so reprehensible and false that they effect your reputation in the community or cast aspersions on you.
Read all about defamation at this link:
http://injury-law.freeadvice.com/libel_and_slander/