With the evidence your son has Kathy, I hope he follows up this travesty legally and publicly.
Maybe they can contact the local newspaper or a larger one, their local news, just something public. Perhaps a lawsuit that they can go public with.
It's not about money, it's principle and hopefully educating others about situations like this; be it the offenders or dog owners. My condolences to all of you for the unnecessary loss of Luke.
Let it never happen again and allow Luke to RIP. Godspeed Luke.