Not a WA lawfag. However, you should send the manager of that Goodwill location a letter immediately, preferably served in such a way that you get confirmation of receipt (such as Certified Mail with a Return Receipt Requested).
The letter should advise that your son was assaulted by a Goodwill employee on x date at x time, describing the employee including name if you have it (should be in the police report or obtainable from the PD). Tell them that you were advised by the manager on duty the day that it happened that they had video surveillance cameras operating that day, and that they are on notice to preserve the evidence for future legal proceedings.
If they don't retain the evidence, they could be liable for spoliation of evidence (that's the legal term for destroying evidence).
As a side note, you should also let the PD know about the video and ask the officer assigned to the case to obtain the video for the criminal investigation. If the cop does get it, you've got a separate source of info. Dunno how the cops are in WA, but the ones I've worked with on the east coast are very good about seeking video evidence and very forthcoming about having it.
You don't actually need to get it right now, you only need to know that you can subpoena it (meaning you need to know who has it.) If the cops get it and Goodwill screws up and loses it - WHOOPS. Break out the checkbook, Goodwill, and make sure to write a goodly number of zeros.