I am sorry you and your dog is going through this difficult situation. Wearing the cone collar is not that bad as many of my dogs have had to wear it for different reasons.  Since your dog is loosing its sight - it is probably time to bring him inside for a more mundane lifestyle.  It is quite obvious that this dog is very special to you and you will do what is right by him. Please let us know how the ERG goes and what you decide to do.