As a long term hedgehog owner, I have encountered many cancers in our pet hedgehogs, I have also spent a great deal of time researching and reading about pet cancers because of it. I had not heard that piroxicam was going off the market though, I wonder why that is. I have read some articles about using NSAIDS for cancer treatment, I have to wonder if one of the others may provide some support.
My Rose was diagnosed with Mast Cell cancer in her lymphatic system this last January. We immediately changed her diet and started adding supplements to help support her nutritionally. Ask doc if a high protein/fat diet with low simple carb would be beneficial for your dog. Also ask about arginine, omega 3 fatty acids, and other supplements you can try. Both of these have had studies done that show they help to slow some cancers.
I don't know much about bladder cancer, but for some cancers prednisone can also help provide quality of time.
Definitely ask about changing his diet and adding supplements. They won't cure him, but they may help support him and give him a better quality of life while he is still with you.