Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that Gideon has developed a large, cystic sarcoma on his back leg which is causing him discomfort and leading to a limp.
Fentanyl (an opiod pain medication like morphine) patches are pretty good at controlling pain, and I am concerned that even with that he is having episodes where he hides and doesn't seem himself. That means that his pain is not well controlled.
We can try adding a steroid or a nonsteroidal anti-inflammatory like Metacam (meloxicam) or Onsior (robenacoxib) for additional relief, but these cannot be given long term, although perhaps intermittent use could be effective, at least for a while. Gabapentin or Amantadine are other additional drugs we can use along with the fentanyl patch to control pain.
If he is an otherwise very healthy fellow and this mass cannot be removed because it is too large then another option is amputation of the affected leg. That may seem drastic, but it will remove the source of his pain and may cure his cancer. Of course before this is done he should have chest radiographs and an abdominal ultrasound to make sure the tumor hasn't spread.
If amputation is not an option, due to other health issues or tumor spread, and we cannot control his pain, then humane euthanasia is an option. I know that is difficult to consider but if his quality of life is deteriorating then it may be the kindest thing for him.
I am sorry that you are struggling with this, please let me know if you have any further questions.