It is never easy to make the decision to say goodbye to a good friend. No one wants to go through the pain of a loss, but you need to think about how your furry friend feels.
Rabbits can tell you how they feel by their body language. How the ears move, how the head is held, the way he sits, stands or lays can all indicate whether your bunny is happy, interested, excited or relaxed. He would have a normal way of greeting you in the morning or when you come home from work. In most cases, he will either jump up on the door or come towards you. When you let him out, he will probably take of exploring.
A bunny that no longer has the zest for life will probably have his ears flat against his head most of the day. Either crouch or lay in his hutch most of the time, sometimes maybe even sit facing the corner of the hutch. When his hutch door is open, he probably will not rush out to explore. Some of these behaviors can be signs of pain that can be treated once the source of the pain is located. Some pains such as from cancers, arthritis, or other internal problems can be treated to some extent, but as the illness progresses, it may not be enough.
You are the only one that can tell if your bunny can no longer live with a debilitating illness. I recently had to euthanize 11 year old One Boot. An inoperable mass developed on his leg. At first, the mass did not seem to affect him. He was still happy and loved to explore and visit with the other bunnies. But it grew quickly until he could no longer use the leg. He started spending most of his days facing the corner or hiding behind his litter box. He had trouble hopping around and no longer explored. Within days, he developed blood in his urine, lost the twinkle and shine in his eyes and lost interest in food, even his favorite dandelion leaves. I knew it was time. Hard as it was for me, I knew One Boot needed to rest and be free.