Are you sure he is not just resting his leg on his back?
If not you need an avian veterinarian to look at him so you need to get him captured. You might try a wildlife rescue facility, a zoo or other facility in your area to see if they can help. You might also try Exotic DVM Linda Harrison in Ft Lauderdale to see if she may have or know someone to help with the rescue. The Regal Swan « Hide