So sorry that poor Sudoku is still struggling. It does sound as if the Tramadol is having an effect, am just wondering where the pain is , gut I assume as his bladder seems to be sorted for the moment.
I am wondering how to make things seem a bit more positive for you and him.
If he is terminally ill then it would be good if his last days/ weeks were not filled with anxiety. Why not try thinking that he is terminally ill rather than trying to get him well, thinking this way will not mean you are giving up on him, but it might make is less stressful for you. Perhaps stop weighing him? Perhaps just decide to feed him enough to keep him comfortable rather than trying to get him to gain weight. Keep offering nice fresh grass and herbs etc. He is eating his hay so he will keep his teeth in check.
I rarely have piggies put to sleep as with correct pain relief and a comfortable familiar environment they can die a natural death with their friends nearby. I only consider euthanasia if they have uncontrollable symptoms. It doesn't sound as if Sudoku is 'suffering', he is miserable, but does have some quality of life?
You really have done everything possible Laura, so rather than keep tormenting yourself, allow yourself to just enjoy his presence and keep showing him all the love you feel for him. Thinking of you xx