12 isn't that old for a small breed dog, and I certainly would not euthanize a dog solely for loss of sight. Most dogs do quite well blind. He will need a fenced area to go out in, or to be leash walked, and not allowed to roam unfettered, as there's a good chance he could get lost or hurt. Otherwise, he should continue to have a good quality of life with minimal accommodation for his loss of sight. My boyfriend's Boston is 13, also one-eyed and 98% blind in the remaining eye. She gets around just fine in the house and yard.