Yes, but until he's feathered and old enough to take care of himself, you don't really want him running in the yard alone (or swimming for that matter). Ducklings are extremely vulnerable to predators and the weather. He can go outside in a sheltered, protected area on a nice day (once he's regained his strength), but remember that he has no mother to take care of him, so you should keep an eye on him.
I mentioned chick starter in the other post (I hadn't even seen this one yet), but if duckling feed is available, that's much better. (Personally, I can't get duck food, only chicken food, so it didn't even cross my mind.) If you have to use chicken food, I think you have to add extra niacin, but I'm not sure how long and how much.