Sorry I have been slow in replying. We have been out of town for a few days. What a nightmare for you! Monty never had a complete amputation, just the tip and he had his tail pocket reconfigured so it wasn't deep anymore. We would have to give him a pretty thorough cleaning everytime he poo'd otherwise he would get infections and it was so much better after the surgery. Unfortunately, he started getting infections to the side (we thought it was his anal gland at first), it would drain a lot and smell bad. We would get it cleared up, then back it would come a few weeks later. After several go-arounds with that it was decided that something inside was causing the deep infection that antibiotics were unable to knock out completely and surgery was needed. Our vet operated on him and followed the course of the infection (several inches and a u-turn!) and found some ingrown hairs!! Something as minor as that caused it all. So far everything is looking good.
Do you have a Veterinary training college anywhere near you? I don't know where in the country you are but you have seen two bully vets with no definitive answer, so I would think about trying something else. We are very lucky here as we have Texas A & M University fairly close and our wonderful vet sent my sons dog there when they felt they couldn't help. If you don't have one within a few hours drive, maybe your vet could send your dogs records and see what they have to say. I really don't know what else to suggest. Others have given you good advice about different foods. I certainly hope you can get this cleared up, poor guy. I know how frustrating it can be and how worried for him you are. Let us know how it goes.