He may have gotten an eye infection that damaged the eyes and is why he was left behind. I'm not sure if he will be able to make it blind even with your help. You could try some antibiotic eye ointment to prevent any further damage in case he can see at all. And give him supportive care of water deep enough to fit his whole bill in, and some extra vitamins like poly-vi-sol without iron it's drops for babies, and brewer's or nutritional yeast in its food. If it can't see to find food or water tho I'm sure you know what that means for his future....I'm so sorry. I commend you on your effort to help a defenseless baby like you did. If he isn't suffering and can get food and water tho if it's an eye infection and you treat it he could possibly recover with enough sight to make it as a domestic, special needs duck.