I would recommend moving him into a small pen with soft bedding, where he can reach food and water without getting up. Then get some poultry electrolytes into him, as he probably won't eat much because of the stress. If you can keep him hydrated and get some calories into him with an electrolyte, that will give him a chance to heal. I've heard Chick start or Nutridrench poultry both work well, others can advise you on that. If he has special treats he likes try feeding those too. I'm sorry, unless you have a local vet that does poultry, it's really hard to get help with an ill or injured duck.