He is a wild Mallard.  In the previous years he and his mate would come and go a couple times a day during the spring and summer.  I don't know where exactly their nest was through they always left my store heading east.  There is now alot of new construction in the area and I'm guessing his nest has probably been destroyed.  We have not seen his mate at all and he has not left the store at all for 3 days now.  I am giving him food and water.  I have duck feed from the livestock supply store- corn, wheat & milo - and I give him chopped veggies, greens, and mealworms as well. 
He seems healthy other than the injury which is not bleeding or infected looking.  He is still plump and gets around OK although I have not seen him fly since he first flew in 3 days ago.  He spends most of the day sitting with his head tucked under his wing but he will come out and eat when ever I coax him to his food bowl.  I'm sure he is lonely without his mate. 
The biggest problem I have now is that my store is owned by a large corporation that requires us to keep up "store standards."  And a duck pooping and spreading food around near the entrance doors is not to their standards. My boss has told me I need to remove the duck but I just don't know how I can do that without stressing him out further or doing him more harm.  I have talked with local veterinarians and animal control but got no help from them.