DR TCB once I not able to get a cat the needed care due to a sick baby. It was gut wrenching for me. When our cat, Bogie, was diagnosed with diabetes we opted not to treat her due to her temperament. Are there pills you can give Nicky the same way you are giving him his pain meds, along with a special diet?
When Bogie was diagnosed with renal disease he tolerated the fluids well but after awhile he did not want any more pills. I would have to show him my hands were empty in order to pet him or he avoided me. That is when I made the decision it was time. I let him/us have a few days free of medications and the side effects and then we said goodbye before he became seriously ill.
I hope this helps you feel better about what you decide. Whatever it is, please do not feel guilty. At his age he shouldn't be hiding to avoid treatment, but he might be fine with it if you want to try it.
Sending vibes of comfort & acceptance.