Speaking from a vet Nurse point of view and as a pet owner (rats, rabbits, guinea pigs) I've had to put alot of my own pets to sleep and it never gets any harder.
These are the things i always asked my self for my own pets, and if you can answer them honestly from the pets point of view then i think you have your anwser:
Can he get around on his own with out distress
Can he eat and Drink
Is his quality of life good
Is he happy
Does he still have dignity
To me i think i know what i would do, but its never an easy choice to make.
Remeber him how he was not how he may become.
I had my bunny with the same and it was very distressing as the disease progressed.
My thoughts are with you
