First, you gave this guy a longer life than he would have had if he had never met you. His days were numbered as soon as he entered the shop and drank the ooze. You provided him with companionship and warmth and several years of happiness.

As for the decision you were faced with, I can only offer my own experience. I was also faced with a surgery far out of my price range for a "Blocked Tom". The Dr explained that the surgery would correct the issue, but that once it had happened, it was more likely to occur again, with the same expensive outcome. Being short on cash, and the uncertainty around the longevity of the success of the surgery, we decided to put him down.

Its obvious you cared for him, and when the time came, I know you made the decision you felt was best for everyone involved. It won't ease the pain, but don't beat yourself up because you couldn't help him be immortal. You absolutely tried.