I'm so sorry they took that beautiful boy away from you! The police are fools, and you should absolutely try to take this all to a higher level. Maybe write to your local newspaper. I don't know what the laws are in your area, but could you maybe file some sort of ownership suit?? With all the vet bills you've accumulated, and food, and shelter, and how long you've had him, and how they just left him there, you could maybe take it to a small claims court and either sue for the money you've put into him or ownership of the dog himself??
My heart goes out to you and Mr. Big right now.
I'm hoping for the best.