Hey OP,

IANAL!

Here is some pertinent info regarding the law the manager thinks he is protected by:

In seeking to avail (protect) themself of the shopkeeper's privilege, the proprietor or agent thereof (store security) must ensure:

The investigation is conducted near or on the premises; the detention itself should be effected either on the store premises or in the immediate vicinity thereof. The privilege likely would not apply to after-the-fact questioning of a suspected thief who had left the store's property. While the common law does permit the owner of goods acting on fresh pursuit to use reasonable force to recapture his or her goods from one who actually took them wrongfully, in doing so the property owner acts at his or her own peril. Moreover, the investigation must be to determine ownership of the property, not to force a confession.

The shopkeeper has reasonable grounds to suspect the particular person detained is shoplifting. (Looking like Gary =/= reasonable ground to suspect a particular person detained is shoplifting)

Only reasonable, nondeadly force is used to effect the detention. Such force being justified when the suspect is in immediate flight or violently resists detention. (We need to find out exactly what went down - clear video would be key)

The detention itself lasts only the time necessary to make a reasonable investigation of the facts. Fifteen minutes may be too long where all that is necessary is to ask the cashier whether the detainee has paid. (Need more facts)

It would be really important that you try to obtain video (if it still exists) of that night. I would contact a lawyer if you have not already.

Definitely consider that your son might not be capable or willing to relay you 100% factual information. Don't act on his information alone - corroborate the story. I have a hunch that he was actually just detained, not arrested, but does not understand that term difference (what 15yo would). Nonetheless, it is still possible the security acted in such a way as to prevent them from protection via shopkeeper privilege. Lawyer lawyer lawyer!!

Best of luck OP.