Got a call from the vet mid-surgery. The tumor was so malignant that it had attached itself to his shoulder-blade, and into the shoulder muscles, and had taken up so much blood flow that all the tissue around it was necrotic. Poor guy must have been in more pain than I ever thought over the last month. There would have been no way to remove it all, and if they'd tried, it would have damaged the nerves and muscle so much that his leg would have likely been useless anyway.
So we made the call to have him put to sleep while he was still unconscious from anesthesia. The vet was kind enough to wait for my wife, daughter, and I to come by and say goodbye, and then I stayed for the final shots. Not how I planned for today to go, and I'm going to miss him like crazy, but I'm grateful for the 10+ years we had with such an amazing dog and glad his pain is over. Thanks again guys, and RIP Mookie, you were such a good boy.