I am so very sorry to read of your boys diagnosis. We have also been down this road and while we opted for amputation and were lucky to get a lot of time with our angel Charlie, when cancer returned I made the brutal decision to let him go. As many have already responded, we have the responsibility to help our pups and care for them like you are doing. Unfortunately the ultimate burden is having to make the decision to set them free when there are no alternatives. Your boy is in a lot of pain based on your description and you have two choices, immediately up his meds and/or add meds such as gabapentin, tramadol, etc or make the decision to set him free from the pain. My heart breaks for you as it's not easy but I know you love your boy and it's time not to be selfish but generous and think of him.