I wouldn't put him down. Give him a fighting chance, If it starts to be too much, maybe consider giving it back to an lfs or an aquarium around you to see if there is something they can do for it. Obviously, it's hard to find food if you can't see it. but, they have their lateral line that detects things and their sense of smell. A question I would ask is how did it lose it's sight? Extreme case of popeye? does he have eyes or are they cloudy from other reasons Parasites etc. That would be a concern if it was the case.