My dog has prostate cancer and extreme pain when pooping. What to do?

I'm sorry Pooh Bear has cancer. It sounds like your vet is doing a good job of trying to treat as best as possible, and I'm assuming the type of cancer diagnosed was transitional cell carcinoma since piroxicam has been prescribed. If that is the case, I have seen these dogs get many more months of good quality life, as long as clinical signs are controlled.

If the type of cancer hasn't been determined and the diagnosis has been made on just the fact that he has an enlarged prostate, then I would caution that dogs can have adenocarcinoma of the prostate, another type of cancer which metastasizes readily and is quite aggressive, and ultimately difficult to treat. It typically does not respond to piroxicam.

Straining to defecate and pain on defecation with prostatic cancers is usually due to the colon becoming trapped between the enlarged prostate and the enlarged sublumbar lymph nodes. If the cancer is a TCC, it should respond to piroxicam, and the prostate should shrink a bit, which will hopefully improve the symptoms. If the cancer is another type of cancer, unfortunately it's unlikely that the current treatment plan is going to help. Stool softeners such as lactulose may help to improve the consistency of the stool, and make it easier to pass. Your vet might also consider adding on gabapentin as an additional pain reliever to the current regimen, as that may add some additional relief.