I did find your comment 'If it was my dog, i'd have anything done to make his life easier, but it's not my dog' rather hard hearted - poor dog cannot help his situation. If you are not prepared to get his weight down and treat his eye and arthritis then rather than having him put down try contacting a Springer breed rescue, or even the Oldies Club, who specialise in rehoming older dogs. They would both have any necessary treatment carried out and find a suitable home for him where he would be loved and appreciated for the rest of his life.