From what I gather, Buster has had no problems moving on. Holding him during the last moments was a great blessing for him, despite the physical agony. It wasn't by accident that he left. He now knows why. He is very joyful and feels very free and thinks of you and your love for him often. Animals also have karmic issues to pass through. His accident was served as a big release of an old karmic burden for him.
I have worked with animal souls and helped them along when stuck. I use sounds.
Andreas